BUG: Fix ts precision issue with groupby and NaT (#19526)
- [X] closes #19526
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19530?src=pr&el=h1) Report
> Merging [#19530](https://codecov.io/gh/pandas-dev/pandas/pull/19530?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/93c86aa13e1b7816c762b4ff372aef80a7830af8?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19530/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19530?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19530      +/-   ##
==========================================
+ Coverage    91.6%    91.6%   +<.01%     
==========================================
  Files         150      150              
  Lines       48750    48750              
==========================================
+ Hits        44657    44658       +1     
+ Misses       4093     4092       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <100%> (ø)` | :arrow_up: |
| #single | `41.75% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19530?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19530/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.17% <100%> (ø)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/19530/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.56% <0%> (+0.06%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19530?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/19530?src=pr&el=footer). Last update [93c86aa...2fb23d6](https://codecov.io/gh/pandas-dev/pandas/pull/19530?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!

keep em coming! thanks! this was a good experience for a first commit.