Mark ASRunLoopQueue as drained if it contains only NULLs (#558)

* Mark ASRunLoopQueue as drained if it contains only NULLs

* Update CHANGELOG.md

* Cover ASRunLoopQueue with tests

* Include PR link in CHANGELOG.md

* Replace license header of ASRunLoopQueueTests.m with correct one

* Insert a nil in _internalQueue to ensure compaction, instead of maintaining a state for _isQueueDrained

Source/ASRunLoopQueue.h

@@ -41,6 +41,8 @@ AS_SUBCLASSING_RESTRICTED
 
 - (void)enqueue:(ObjectType)object;
 
+@property (nonatomic, readonly) BOOL isEmpty;
+
 @property (nonatomic, assign) NSUInteger batchSize;           // Default == 1.
 @property (nonatomic, assign) BOOL ensureExclusiveMembership; // Default == YES.  Set-like behavior.