Change defaults for rolling/expanding.apply engine kwargs to None
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This change was supposed to be apart of https://github.com/pandas-dev/pandas/pull/35182 but was overlooked. Since 1.1 isn't out, this doesn't need to be backported correct?

thanks @mroeschke 