This got lost during the QStringView port that happend in Qt 6.0
(commit 548dcef089) because
QString::operator+=(QStringView) does not copy the nullness of the right
side, whereas QString::operator+=(const QString &) does.
Pick-to: 6.2 6.4 6.5
Fixes: QTBUG-84315
Change-Id: Ide4dbd0777a44ed0870efffd17399b772d34fd55
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>