Windows QPA: Do not play sound for QMessageBox::NoIcon

Amends 2f366a63b2.

Task-number: QTBUG-82682
Task-number: QTBUG-81342
Change-Id: I30f465bf432e27828db460f6dbbb59eee0cca8f2
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: André de la Rocha <andre.rocha@qt.io>

src/plugins/platforms/windows/uiautomation/qwindowsuiaaccessibility.cpp

@@ -105,6 +105,7 @@ static QString alertSound(const QObject *object)
         case Critical:
             return QStringLiteral("SystemHand");
         }
+        return QString();
     }
     return QStringLiteral("SystemAsterisk");
 }