BUG: NaT+Period doesnt match Period+NaT
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Most of the edits in `__add__` and `__sub__` are just an un-indent to get rid of an unreachable branch, but found this incorrect case along the way.