Revert "Fix the stub for layerActionForKey: to let CA continue the se…

…arch (TextureGroup#1441)"

This reverts commit bf466e8.

Source/ASDisplayNode.mm

@@ -92,10 +92,7 @@ BOOL ASDisplayNodeNeedsSpecialPropertiesHandling(BOOL isSynchronous, BOOL isLaye
 void StubImplementationWithNoArgs(id receiver, SEL _cmd) {}
 void StubImplementationWithSizeRange(id receiver, SEL _cmd, ASSizeRange sr) {}
 void StubImplementationWithTwoInterfaceStates(id receiver, SEL _cmd, ASInterfaceState s0, ASInterfaceState s1) {}
-
-/// Returning nil here won't trigger unwanted default actions, because we override
-/// +defaultActionForKey: to return kCFNull.
-id StubLayerActionImplementation(id receiver, SEL _cmd, NSString *key) { return nil; }
+id StubLayerActionImplementation(id receiver, SEL _cmd, NSString *key) { return (id)kCFNull; }
 
 /**
  *  Returns ASDisplayNodeFlags for the given class/instance. instance MAY BE NIL.