BUG: Fix groupby nunique with NaT
- [x] closes #17575
- [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/17624?src=pr&el=h1) Report
> Merging [#17624](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8276a420a36c26eaab38856177023cb064963f19?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17624/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17624      +/-   ##
==========================================
- Coverage    91.2%   91.18%   -0.02%     
==========================================
  Files         163      163              
  Lines       49637    49640       +3     
==========================================
- Hits        45269    45263       -6     
- Misses       4368     4377       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.97% <100%> (ø)` | :arrow_up: |
| #single | `40.18% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.23% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17624?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/17624?src=pr&el=footer). Last update [8276a42...800d1ad](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=h1) Report
> Merging [#17624](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8276a420a36c26eaab38856177023cb064963f19?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17624/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17624      +/-   ##
==========================================
- Coverage    91.2%   91.18%   -0.02%     
==========================================
  Files         163      163              
  Lines       49637    49655      +18     
==========================================
+ Hits        45269    45276       +7     
- Misses       4368     4379      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.97% <100%> (ø)` | :arrow_up: |
| #single | `40.17% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.23% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `80.37% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `92.83% <0%> (+0.24%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17624?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/17624?src=pr&el=footer). Last update [8276a42...29d4cf6](https://codecov.io/gh/pandas-dev/pandas/pull/17624?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback Thanks for your review. All is now fixed! thanks @Licht-T nice patch! keep em coming!