SAS validate null dates
- [x] closes #39725 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

cc @bashtage  It looks good aside from my personal preference over multi-line lambda functions which are really hard to read.