Add support for run loop queues with no handler, optimize

AsyncDisplayKit/ASRunLoopQueue.h

@@ -16,8 +16,19 @@ NS_ASSUME_NONNULL_BEGIN
 
 @interface ASRunLoopQueue<ObjectType> : NSObject
 
+/**
+ * Create a new queue with the given run loop and handler.
+ *
+ * @param runloop The run loop that will drive this queue.
+ * @param handlerBlock An optional block to be run for each enqueued object.
+ *
+ * @discussion You may pass @c nil for the handler if you simply want the objects to
+ * be retained at enqueue time, and released during the run loop step. This is useful
+ * for creating a "main deallocation queue", as @c ASDeallocQueue creates its own 
+ * worker thread with its own run loop.
+ */
 - (instancetype)initWithRunLoop:(CFRunLoopRef)runloop
-                     andHandler:(void(^)(ObjectType dequeuedItem, BOOL isQueueDrained))handlerBlock;
+                     andHandler:(nullable void(^)(ObjectType dequeuedItem, BOOL isQueueDrained))handlerBlock;
 
 - (void)enqueue:(ObjectType)object;