BUG: Issues with DatetimeTZ values in where and combine_first (#21469 + #21546)
- [ ] closes #21469
- [ ] closes #21546
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

CC: @jreback. Replacement for PR #21544; fixes for #21469 (`Series.combine_first` fails on tz values input) and #21546 (`where` fails on tz values input). Series.combine_first now delegates to `where` for the bulk of its operations.