Test that nan value counts are included when dropna=False. GH#31944
- [x] closes #31944
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Added test per @Dr-Irv request.

Hi y'all, any other changes for this pull request? thanks @icanhazcodeplz 