Bug in to_datetime raising ValueError with None and NaT and more than 50 elements
- [x] closes #39882
- [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

None and NaT are different for unique while convert_listlike casts None to NaT, hence causing dups
Not sure if we could do something better.
@jreback greenish thanks @phofl 