[null-safety] fix Scene.toImage declaration (#21983)

lib/ui/compositing.dart

@@ -37,7 +37,7 @@ class Scene extends NativeFieldWrapperClass2 {
     );
   }
 
-  String _toImage(int width, int height, _Callback<_Image> callback) native 'Scene_toImage';
+  String? _toImage(int width, int height, _Callback<_Image> callback) native 'Scene_toImage';
 
   /// Releases the resources used by this scene.
   ///