TST: Add test for groupby with datetime and NaT values
- [x] closes #35202
- [x] [Tests added and passed]
- [x] All [code checks passed]
- [ ] Added [type annotations]
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @asharmalik19 