BUG: Ignore division by 0 when merging empty dataframes (#17776)
- [x] closes #17776
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=h1) Report
> Merging [#17846](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7e159aeec1d5d2e79311ca688c0e594f0fd5f709?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17846/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17846      +/-   ##
==========================================
- Coverage   91.25%   91.21%   -0.05%     
==========================================
  Files         163      163              
  Lines       50038    50040       +2     
==========================================
- Hits        45661    45642      -19     
- Misses       4377     4398      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.26% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=footer). Last update [7e159ae...ff8e1bb](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=h1) Report
> Merging [#17846](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7e159aeec1d5d2e79311ca688c0e594f0fd5f709?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17846/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17846      +/-   ##
==========================================
- Coverage   91.25%   91.21%   -0.05%     
==========================================
  Files         163      163              
  Lines       50038    50040       +2     
==========================================
- Hits        45661    45642      -19     
- Misses       4377     4398      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.26% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=footer). Last update [7e159ae...ff8e1bb](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=h1) Report
> Merging [#17846](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7e159aeec1d5d2e79311ca688c0e594f0fd5f709?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17846/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17846      +/-   ##
==========================================
- Coverage   91.25%   91.21%   -0.05%     
==========================================
  Files         163      163              
  Lines       50038    50040       +2     
==========================================
- Hits        45661    45642      -19     
- Misses       4377     4398      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.26% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.28% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=footer). Last update [7e159ae...ff8e1bb](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=h1) Report
> Merging [#17846](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/674fb96b33c07c680844f674fcdf0767b6e3c2f9?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17846/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17846      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.02%     
==========================================
  Files         163      163              
  Lines       49801    49802       +1     
==========================================
- Hits        45496    45487       -9     
- Misses       4305     4315      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.79% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.29% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17846/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.61% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=footer). Last update [674fb96...138d886](https://codecov.io/gh/pandas-dev/pandas/pull/17846?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 need you to rebase on master

```
git rebase -i upstream/master
```

then fix any conflicts

```
git push yourupstream yourbranch -f
``` @jreback, thanks, I've made the changes. How can I address the failed AppVeyor build? This test passed when I ran `pytest pandas` on my computer:

```
================================== FAILURES ===================================
_______________________ TestDataFrameAnalytics.test_sum _______________________
[gw0] win32 -- Python 2.7.14 C:\Miniconda3_64\envs\pandas\python.exe
self = <pandas.tests.frame.test_analytics.TestDataFrameAnalytics object at 0x00000000089BB518>
    def test_sum(self):
        self._check_stat_op('sum', np.sum, has_numeric_only=True)
    
        # mixed types (with upcasting happening)
        self._check_stat_op('sum', np.sum,
                            frame=self.mixed_float.astype('float32'),
                            has_numeric_only=True, check_dtype=False,
>                           check_less_precise=True)
pandas\tests\frame\test_analytics.py:449: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas\tests\frame\test_analytics.py:753: in _check_stat_op
    check_less_precise=check_less_precise)
pandas\util\testing.py:1272: in assert_series_equal
    obj='{obj}'.format(obj=obj))
pandas\_libs\testing.pyx:59: in pandas._libs.testing.assert_almost_equal
    cpdef assert_almost_equal(a, b,
pandas\_libs\testing.pyx:173: in pandas._libs.testing.assert_almost_equal
    raise_assert_detail(obj, msg, lobj, robj)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
obj = 'Series', message = 'Series values are different (25.0 %)'
left = '[-13.4449, 0.13886, 9.09849, -0.00019373]'
right = '[-13.4448623657, 0.13885974884, 9.098487854, -0.000193253159523]'
diff = None
    def raise_assert_detail(obj, message, left, right, diff=None):
        if isinstance(left, np.ndarray):
            left = pprint_thing(left)
        if isinstance(right, np.ndarray):
            right = pprint_thing(right)
    
        msg = """{obj} are different
    
    {message}
    [left]:  {left}
    [right]: {right}""".format(obj=obj, message=message, left=left, right=right)
    
        if diff is not None:
            msg += "\n[diff]: {diff}".format(diff=diff)
    
>       raise AssertionError(msg)
E       AssertionError: Series are different
E       
E       Series values are different (25.0 %)
E       [left]:  [-13.4449, 0.13886, 9.09849, -0.00019373]
E       [right]: [-13.4448623657, 0.13885974884, 9.098487854, -0.000193253159523]
pandas\util\testing.py:1089: AssertionError
 1 failed, 15133 passed, 790 skipped, 13 xfailed, 1 xpassed in 517.30 seconds =
Command exited with code 1
``` can you rebase and add a note in 0.21.1 thanks @yeemey 