BUG: Groupby.agg with timegrouper, nat, and numba
- [ ] closes #xxxx
- [x] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

IIRC you did a similar bug fix quite recently (for non-numba?) can you add this pr number to that note. > can you add this pr number to that note

done + green