BUG: None converted to NaN after groupby first and last
- [x] closes #32800
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This PR solves #32800. In #32124 `not checknull()` was introduced to `groupfirst()` and `grouplast()`. `None` is not caught in this conditional and it's being converted to `nan` because of that.

I'm a newbie and not sure whether I should add whatsnew entry or not. Should I?