OSX: Fix warning causing failing tests

Fixes following warning on OS X.

    QtWarning: Failed to get QCocoaScreen for NSObject(0x0)

Signed-off-by: Lukas Holecek <hluk@email.cz>

src/platform/mac/foregroundbackgroundfilter.mm

@@ -103,6 +103,9 @@ bool isNormalAppWindow(QWindow *window)
 
         // Allow windows to pop up on current space.
         const auto wid = window->winId();
+        if (!wid)
+            return false;
+
         auto nsView = reinterpret_cast<NSView*>(wid);
         auto nsWindow = [nsView window];
         if (nsWindow)