iOS: Fix screen size detection in plugins

initWithWindowScene appears to create a window with no area if the
provided scene is null. As a fallback, creating a window without an
associated scene still returns usable bounds.

modules/juce_gui_basics/native/juce_Windowing_ios.mm

@@ -613,7 +613,9 @@ struct TemporaryWindow
         if (@available (ios 13, *))
         {
             SharedResourcePointer<WindowSceneTracker> windowSceneTracker;
-            return [[UIWindow alloc] initWithWindowScene: windowSceneTracker->getWindowScene()];
+
+            if (auto* scene = windowSceneTracker->getWindowScene())
+                return [[UIWindow alloc] initWithWindowScene: scene];
         }
 
         return [[UIWindow alloc] init];