BUG: fixes indexing with monotonic decreasing DTI (#19362)
- [x] closes #19362
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Seems that #19362 is caused by the fact that `np.any([0])` is false. Here's a suggested solution. Also added a test.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20677?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@8756f55`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20677/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20677?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20677   +/-   ##
=========================================
  Coverage          ?   91.81%           
=========================================
  Files             ?      153           
  Lines             ?    49279           
  Branches          ?        0           
=========================================
  Hits              ?    45247           
  Misses            ?     4032           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <100%> (?)` | |
| #single | `41.9% <100%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20677?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/20677/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.72% <100%> (ø)` | |

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[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20677?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/20677?src=pr&el=footer). Last update [8756f55...ecff6e4](https://codecov.io/gh/pandas-dev/pandas/pull/20677?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks for the helpful comments @jreback, I think the updated commit should address the points. https://github.com/pandas-dev/pandas/pull/20677/commits/3c4f38b005c7d7a4e2e66d180a536acc2b9c57a3 was all green @jreback  thanks @mapehe!  Np!