fix opacity layer

com.unity.uiwidgets/Runtime/rendering/layer.cs

         internal override void addToScene(SceneBuilder builder, Offset layerOffset = null) {
             layerOffset = layerOffset ?? Offset.zero;
 
-            bool enabled = true;
+            bool enabled = firstChild != null;
-                enabled = !D.debugDisableOpacityLayers;
+                enabled = enabled && !D.debugDisableOpacityLayers;
                 return true;
             });