Adds a few missing parts of the conversion from QVariant to QJsonValue
after the introduction of the nullptr QVariant. The conversion the other
way is already implemented.
Change-Id: I8b25dec4b476c4761c5098a60944ff11c36f8bec
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>