TST/bug: add test for NaT in Categorical.isin with empty string
- [x] closes #36550
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.


This bug was present in pandas v1.5, but is not present in v2.0.rc1. I added a test for this working as expected now. 
> This bug was present in pandas v1.5, but is not present in v2.0.rc1. I added a test for this working as expected now.

Thanks. Looking at the date of the issue, the bug was present before 1.5.

Things may have changed since I took a break, but we don't normally add a release note when we just add a regression test.

Any idea which PR fixed the issue? No whatsnew note, this is still correct > Any idea which PR fixed the issue?

No, I just tried  the code snippet in the issue and saw that the problem is gone. Then I tried it in v1.5, where it was still present :-).

In v1.5 I also got a warning, when I ran the snippet (`FutureWarning: Inferring timedelta64[ns] from data containing strings is deprecated and will be removed in a future version. To retain the old behavior explicitly pass Series(data, dtype=timedelta64[ns])`), so my guess is whatever fixed that warning, also fixed this, but that's just a guess.

I could do a bisect, but IMO that's probably not worth it...
 > I could do a bisect, but IMO that's probably not worth it...

no problem. The reason I asked is that the PR that fixed may have had a release note and the issue number could have been appended instead of adding the release note here. Yeah, I looked through the whatsnew to find canditates and found several candidates and figured it wasn't worth it to figure which one it was...

I've removed the whatsnew entry in the newest commit. Thanks @topper-123 