-actionForLayer:forKey: of ASDisplayNode should return nil to avoid the node being animated by CA when it's being moved from one supernode to another (#3146)

This reverts commit 1d21c0bd55 introduced in #3063.

Source/ASDisplayNode.mm

@@ -2312,7 +2312,7 @@ static const char *ASDisplayNodeDrawingPriorityKey = "ASDrawingPriority";
   }
 
   ASDisplayNodeAssert(_flags.layerBacked, @"We shouldn't get called back here unless we are layer-backed.");
-  return nil;
+  return (id)kCFNull;
 }
 
 #pragma mark - Error Handling