Applied nitpicks in PromiseQueue shared queue fix.

Set PromiseQueue.prototype._queue to null to avoid confusion.
Removed unneeded _curPromise initialization in constructor.

Signed-off-by: jayther <jayther@gmail.com>

Author: jayther

src/utils/Async.js

@@ -425,7 +425,6 @@ define(function (require, exports, module) {
      */
     function PromiseQueue() {
         this._queue = [];
-        this._curPromise = null;
     }
 
     /**
@@ -434,7 +433,7 @@ define(function (require, exports, module) {
      * The queue of operations to execute sequentially. Note that even if this array is empty, there might
      * still be an operation we need to wait on; that operation's promise is stored in _curPromise.
      */
-    PromiseQueue.prototype._queue = [];
+    PromiseQueue.prototype._queue = null;
 
     /**
      * @private