[Validator] Mention IsNull in Basic Constraints

[wkania]

Email, Date, File and other string constraints have note:

As with most of the other constraints, null and empty strings are considered valid values. This is to allow them to be optional values. If the value is mandatory, a common solution is to combine this constraint with NotBlank.

This pull request adds a note about null to Basic Constraints. Empty/Blank string is an invalid value for them. That's why I added a new note.

[OskarStark]

Looks like a good addition to me 👍

[javiereguiluz]

Yes, I agree this is better. Thanks!