BUG: Series.combine_first with datetime-tz dtype (#21469)
- [x] closes #21469 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Closing this PR. Will open a new one that attempts to fix #21546 in addition to #21469,