BUG: Ensure that fill_na in Categorical only replaces null values
- [x] closes #26215
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Can you add the test from https://github.com/pandas-dev/pandas/pull/27932#discussion_r314318140?

We've had issues in the past with unordered categories where the categories / codes didn't line up. Sure :) I've added a test where the fill value is a series with the same categories as the original series but in a different order, sorry for having missed this the first time Looks great @MarcoGorelli. Thanks!