QMimeData: return null string for null byte array

This is a cherry modification of 5826a7ad921930393c84b790123f493e26d0685c
in 5.15 branch.

Fixes: QTBUG-102438
Pick-to: 6.3 6.2
Change-Id: I8bdb7cb2537c6df7bb1381871b3cc2446ff1d270
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qmimedata.cpp

@@ -160,6 +160,8 @@ QVariant QMimeDataPrivate::retrieveTypedData(const QString &format, QMetaType ty
         switch (typeId) {
         case QMetaType::QString: {
             const QByteArray ba = data.toByteArray();
+            if (ba.isNull())
+                return QVariant();
             if (format == "text/html"_L1) {
                 auto encoding = QStringConverter::encodingForHtml(ba);
                 if (encoding) {