Implements isNull for QVariants of a nullptr so they always return
true to isNull(), instead of depending on how they were constructed.
Task-number: QTBUG-58296
Change-Id: Ibddec795cdadedef7e17d22c265c29e752d8f99f
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>