BUG: maybe_convert_objects with timedelta64 nat
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

user facing at all? > user facing at all?

not AFAICT.  discovered while trying to simplify our casting code by passing convert_timedelta/convert_datetime=True and skipping some of the casting calls.