Fix pd.NaT - Series
Tests will be added after #19139 is merged, since largely that will involve removing an "xfail" from tests that already exist there.

- [x] closes #19158
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

#19139 merged, so rebase and remove xfails. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19251?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@7208610`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19251/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19251?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19251   +/-   ##
=========================================
  Coverage          ?   91.54%           
=========================================
  Files             ?      147           
  Lines             ?    48825           
  Branches          ?        0           
=========================================
  Hits              ?    44699           
  Misses            ?     4126           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.92% <ø> (?)` | |
| #single | `41.69% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19251?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/19251?src=pr&el=footer). Last update [7208610...dbb2a82](https://codecov.io/gh/pandas-dev/pandas/pull/19251?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. Ping thanks!