ENH: Categorical.fillna allow Categorical/ndarray
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

xref #32414.

cc @TomAugspurger ATM the new test implemented in tests.arrays.categorical.test_missing is failing on the ndarray case for reasons that I dont fully grok.  Are you familiar with this?