CLN/TST: delegate StringArray.fillna() to parent class + add tests
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Does this need a whatsnew? > Does this need a whatsnew?

I don't think it is necessarily needed, it's only a change in the error message I think? I pared down this PR to just the fillna test for strings. Discussion on `_validate_setitem_value` is housed in #36226 (happy to work on that in a follow-on unless somebody else wants to) thanks @arw2019 