This method restores the Qt 5 logic for testing for null variants,
where for core Qt types the isNull check is forwarded to the actual
data type.
E.g. on Qt 5: QVariant( QDateTime()).isNull() is true, but
on Qt 6 it's false
This breaks a LOT of assumptions made throughout QGIS. The new
helper method avoids this breakage by ensuring we follow the Qt 5
logic also on Qt 6 builds, and also gives us the option to extend
this logic for user types (e.g QVariant( QgsGeometry() ).isNull() could
return true). (That's not included here)
