BUG: na parameter for str.startswith and str.endswith not propagating for Series with categorical dtype
- [x] closes #36241
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I parametrized dtype in existing tests. Should I create a separate test instead?
thanks @asishm  @meeseeksdev backport 1.1.x