BUG: Don't raise for NDFrame.mask with nullable boolean
- [x] closes https://github.com/pandas-dev/pandas/issues/35429
- [x] closes https://github.com/pandas-dev/pandas/issues/36395
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

~(Need to wait until the v1.1.3 whatsnew gets merged before adding a release note.)~

This is taking the approach of treating NA as False within NDFrame.mask and NDFrame.where when dealing with a nullable boolean mask.