Don't add empty OpacityLayer to the engine (#31520)

Fixes https://github.com/flutter/flutter/issues/31517

packages/flutter/lib/src/rendering/layer.dart

@@ -1242,6 +1242,9 @@ class TransformLayer extends OffsetLayer {
 /// When debugging, setting [debugDisableOpacityLayers] to true will cause this
 /// layer to be skipped (directly replaced by its children). This can be helpful
 /// to track down the cause of performance problems.
+///
+/// Try to avoid an [OpacityLayer] with no children. Remove that layer if
+/// possible to save some tree walks.
 class OpacityLayer extends ContainerLayer {
   /// Creates an opacity layer.
   ///
@@ -1281,9 +1284,9 @@ class OpacityLayer extends ContainerLayer {
 
   @override
   ui.EngineLayer addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
-    bool enabled = true;
+    bool enabled = firstChild != null;  // don't add this layer if there's no child
     assert(() {
-      enabled = !debugDisableOpacityLayers;
+      enabled = enabled && !debugDisableOpacityLayers;
       return true;
     }());
     if (enabled)

packages/flutter/test/widgets/opacity_test.dart

@@ -183,4 +183,14 @@ void main() {
       skip: !Platform.isLinux,
     );
   });
+
+  testWidgets('empty opacity does not crash', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      RepaintBoundary(child: Opacity(opacity: 0.5, child: Container())),
+    );
+    final Element element = find.byType(RepaintBoundary).first.evaluate().single;
+    // The following line will send the layer to engine and cause crash if an
+    // empty opacity layer is sent.
+    await element.renderObject.layer.toImage(Rect.fromLTRB(0.0, 0.0, 1.0, 1.0));
+  });
 }