[null-safety] fix types of layer code (#21959)

lib/ui/compositing.dart

@@ -538,7 +538,7 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
     return layer;
   }
 
-  EngineLayer _pushShaderMask(
+  void _pushShaderMask(
       EngineLayer engineLayer,
       Shader shader,
       double maskRectLeft,
@@ -587,7 +587,7 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
     return layer;
   }
 
-  EngineLayer _pushPhysicalShape(EngineLayer outEngineLayer, Path path, double elevation, int color, int shadowColor,
+  void _pushPhysicalShape(EngineLayer outEngineLayer, Path path, double elevation, int color, int shadowColor,
       int clipBehavior) native 'SceneBuilder_pushPhysicalShape';
 
   /// Ends the effect of the most recently pushed operation.