QNX: Fixed issue with non Qt-related mm-renderer windows

Windowgrabber would catch events from windows that were not related to
Qt Media Playback. Now there is a check to ensure the window id to
compare against is set before assigning the window to the Windowgrabber.

Change-Id: Ic85198de5fdb05e9fa740fc7b3b81f3bdffd631d
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: James McDonnell <jmcdonnell@qnx.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>

src/plugins/qnx/common/windowgrabber.cpp

@@ -276,7 +276,8 @@ bool WindowGrabber::handleScreenEvent(screen_event_t screen_event)
         return false;
     }
 
-    if (m_windowId == idString) {
+    // Grab windows that have a non-empty ID string and a matching window id to grab
+    if (idString[0] != '\0' && m_windowId == idString) {
         m_window = window;
         start();
     }