BUG: Series.rank modifies inplace with NaT
- [x] closes #18521
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Looks like `_ensure_data` converts NaT to -9223372036854775808 then that gets converted back to datetime64.