Implement pd.NA checking function
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This may already exist in some form, but if not I think it could be useful to have a fast function for checking specifically for the presence of pd.NA (e.g. if we're looking at an arbitrary array and don't know if we'll have access to a _mask attribute, or if we're handling some other data type that uses pd.NA for missing values but doesn't have this attribute).