BUG: GroupBy.apply with timegrouper and NaT
- [ ] 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

I'm increasingly sure that we should use result_index instead of all of the various near-identical alternatives in these files.  The hard part is finding test cases where the distinction is relevant.  This gets two of them.