QNX: Do not create default platform theme

If the theme name is not recognized we should not create a QPlatformTheme, but rather return 0.
If no platform theme can be created on creation of the QGuiApplication,a default QPlatformTheme
will be created automatically.

Change-Id: I685fa352c87d74c225c7f91c10cb84fb5128a3f6
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>

src/plugins/platforms/qnx/qqnxintegration.cpp

@@ -470,7 +470,7 @@ QPlatformTheme *QQnxIntegration::createPlatformTheme(const QString &name) const
     qIntegrationDebug() << Q_FUNC_INFO << "name =" << name;
     if (name == QBlackberryTheme::name())
         return new QBlackberryTheme(this);
-    return QPlatformIntegration::createPlatformTheme(name);
+    return 0;
 }
 #endif