touch up and skip test on html and firefox

lib/web_ui/lib/src/engine/canvaskit/layer_visitor.dart

@@ -600,7 +600,7 @@ class PaintVisitor extends LayerVisitor {
       final List<PictureLayer>? pictureChildren =
           picturesUnderShaderMask[shaderMask];
       if (pictureChildren != null) {
-        for (final PictureLayer picture in pictureChildren!) {
+        for (final PictureLayer picture in pictureChildren) {
           canvases.add(viewEmbedder!.getOptimizedCanvasFor(picture));
         }
       }

lib/web_ui/test/ui/scene_builder_test.dart

@@ -400,7 +400,9 @@ Future<void> testMain() async {
 
       await matchGoldenFile('scene_builder_shader_mask_clipped_out.png',
           region: region);
-    });
+    },
+        skip: isFirefox &&
+            isHtml); // https://github.com/flutter/flutter/issues/86623
   });
 }