Add emptyAsUndefined and possibly emptyAsNull props to form fields

[Tobbe]

Optional form fields whose name ends in Id are treated as DB field relation input fields and when they're empty we automatically convert '' to undefined. We want it to be possible to override this behavior by setting emptyAsUndefined={false} and also to get this behavior on other fields, whose name don't end with Id, by setting emptyAsUndefined={true}

Also, because null is somewhat of a special value for DBs it might make sense to add emptyAsNull as another option.

For more context:
PR: #4302
Issue: #4297