BUG: datetime64[us] arrays with NaT cannot be cast to DatetimeIndex
- [x] closes #9114
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixed by #13692. Added tests.

Very nice -- thanks!
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13770?src=pr) is 84.56% (diff: 100%)

> No coverage report found for **master** at b60e42b.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last update [b60e42b...6ae1066](https://codecov.io/gh/pydata/pandas/compare/b60e42b02040624f319cadd60a99fe50eb62d62e...6ae1066a48d0bf76d6c218176634708e962f35f6?src=pr)
 thank you sir!
