Adapt SQL drivers to Qt 6 change of QVariant::isNull

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>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/plugins/sqldrivers/mysql/qsql_mysql.cpp b/src/plugins/sqldrivers/mysql/qsql_mysql.cpp
index caf406da46..837805cc46 100644
--- a/src/plugins/sqldrivers/mysql/qsql_mysql.cpp
+++ b/src/plugins/sqldrivers/mysql/qsql_mysql.cpp
@@ -929,7 +929,7 @@ bool QMYSQLResult::exec()
 
             MYSQL_BIND* currBind = &d->outBinds[i];
 
-            nullVector[i] = static_cast<my_bool>(val.isNull());
+            nullVector[i] = static_cast<my_bool>(QSqlResultPrivate::isVariantNull(val));
             currBind->is_null = &nullVector[i];
             currBind->length = 0;
             currBind->is_unsigned = 0;