datetimelike indexes add/sub zero-dim integer arrays 
Setup:
```
dti = pd.date_range('2016-01-01', periods=3, freq='H')
one = np.array(1)
```

0.21.1:
```
>>> dti + one
DatetimeIndex(['2016-01-01 00:00:00.000000001',
               '2016-01-01 01:00:00.000000001',
               '2016-01-01 02:00:00.000000001'],
              dtype='datetime64[ns]', freq='H')
>>> dti.freq = None
>>> dti + one
DatetimeIndex(['2016-01-01 00:00:00.000000001',
               '2016-01-01 01:00:00.000000001',
               '2016-01-01 02:00:00.000000001'],
              dtype='datetime64[ns]', freq=None)
```

Master:  (See #19011)
```
>>> dti + one
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/indexes/datetimelike.py", line 685, in __add__
    elif is_offsetlike(other):
  File "pandas/core/dtypes/common.py", line 294, in is_offsetlike
    elif (is_list_like(arr_or_obj) and len(arr_or_obj) and
TypeError: len() of unsized object
```

After
```
>>> dti + one
DatetimeIndex(['2016-01-01 01:00:00', '2016-01-01 02:00:00',
               '2016-01-01 03:00:00'],
              dtype='datetime64[ns]', freq='H')
>>> dti.freq = None
>>> dti + one
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/indexes/datetimelike.py", line 683, in __add__
    return self.shift(other)
  File "pandas/core/indexes/datetimelike.py", line 821, in shift
    raise ValueError("Cannot shift with no freq")
ValueError: Cannot shift with no freq
```

- [x] closes #19012
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Wow, yah, just calling `other = lib.item_from_zero_dim(other)` at the top of `__sub__` and `__add__` is _way_ simpler. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19013?src=pr&el=h1) Report
> Merging [#19013](https://codecov.io/gh/pandas-dev/pandas/pull/19013?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0e3c797c4c12fa04fd745e595e822886e917b316?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19013/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19013?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19013      +/-   ##
==========================================
+ Coverage   91.55%   91.57%   +0.02%     
==========================================
  Files         150      150              
  Lines       48939    48941       +2     
==========================================
+ Hits        44805    44817      +12     
+ Misses       4134     4124      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.93% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.75% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19013?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19013/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.05% <100%> (+0.01%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19013/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.74% <0%> (-0.22%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19013/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19013?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/19013?src=pr&el=footer). Last update [0e3c797...67522b3](https://codecov.io/gh/pandas-dev/pandas/pull/19013?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!