BUG: Series.resample fails on NaT index
- [x] closes #39227
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

This will also resolve the core issue of https://github.com/pandas-dev/pandas/pull/34998#discussion_r556463043. cc @jorisvandenbossche 

The added test:

```
    s = series.copy()
    s.index = PeriodIndex([NaT] * len(s), freq=freq)
    result = getattr(s.resample(freq), resample_method)()
```

currently fails for `freq="M"`, where upsample is used instead of downsample leading to failure since the index is not unique. I can't tell if this is a bug or an operation that should raise. It also fails for `DataFrame`, so I think is unrelated to the issue here.

cc @jbrockmendel 