BUG: Inconsistent return type for downsampling on resample of empty DataFrame
 - [x] closes #14962
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

update this test as well (IOW add size)
```
def test_resample_empty_dataframe(self):
        # GH13212
        index = self.create_series().index[:0]
        f = DataFrame(index=index)

        for freq in ['M', 'D', 'H']:
            # count retains dimensions too
            methods = downsample_methods + ['count']
            for method in methods:
                result = getattr(f.resample(freq), method)()

                expected = f.copy()
                expected.index = f.index._shallow_copy(freq=freq)
                assert_index_equal(result.index, expected.index)
                self.assertEqual(result.index.freq, expected.index.freq)
                assert_frame_equal(result, expected, check_dtype=False)
``` # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=h1) Report
> Merging [#15093](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/344cec77f1904343f8d253d7cfd8d5fccddb08de?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15093/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15093      +/-   ##
==========================================
- Coverage   90.95%   90.92%   -0.03%     
==========================================
  Files         161      161              
  Lines       49276    49282       +6     
==========================================
- Hits        44817    44811       -6     
- Misses       4459     4471      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.68% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.18% <42.85%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.13% <100%> (+0.03%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=footer). Last update [344cec7...44f52be](https://codecov.io/gh/pandas-dev/pandas/pull/15093?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback could you please take a look, that's solution is okay or not? can you rebase / update can you rebase / update @jreback  @discort sorry been MIA on this one. can you rebase. can you rebase can you rebase and update? @jreback  @jreback  thanks!