In Qt 5, QVariant::isNull returned true if either the variant didn't
contain a value, or if the value was of a nullable type where the type's
isNull member function returned true.
In Qt 6, QVariant::isNull only returns true for variants that don't
contain a value; if the value contained is e.g. a null-QString or
QDateTime, then QVariant::isNull returns false.
This change requires a follow up in the SQL drivers, which must
still treat null-values the same as null-variants, lest they write data
into the data base.
Add a static helper to QSqlResultPrivate that implements isNull-checking
of variants that contain a nullable type relevant for Sql, and add a
test case to the QSqlQuery test that exercises that code.
Pick-to: 6.2 6.3
Fixes: QTBUG-99408
Fixes: QTBUG-98471
Change-Id: I08b74a33aa3235c37d974f182da1f2bdcfd8217e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
999d856bc8