Timeline: return 'null' from JSCallStack::asTraceFormat() when stack is absent

Source/devtools/front_end/timeline/TracingTimelineModel.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @param {!WebInspector.TracingModel} tracingModel
  * @constructor
  * @extends {WebInspector.TimelineModel}
  */
 WebInspector.TracingTimelineModel = function(tracingModel)
@@ skipping 292 matching lines @@
             event.selfTime = duration;
             eventStack.push(event);
         }
 
         if (this._currentScriptEvent && event.startTime > this._currentScriptEvent.endTime)
             this._currentScriptEvent = null;
 
         switch (event.name) {
         case recordTypes.CallStack:
             var lastMainThreadEvent = this._mainThreadEvents.peekLast();
-            if (lastMainThreadEvent)
+            if (lastMainThreadEvent && event.args.stack && event.args.stack.length)
                 lastMainThreadEvent.stackTrace = event.args.stack;
             break;
 
         case recordTypes.ResourceSendRequest:
             this._sendRequestEvents[event.args.data["requestId"]] = event;
             event.imageURL = event.args.data["url"];
             break;
 
         case recordTypes.ResourceReceiveResponse:
         case recordTypes.ResourceReceivedData:
@@ skipping 319 matching lines @@
     {
         var tokens = [this.title()];
         var data = this._event.args.data;
         if (data) {
             for (var key in data)
                 tokens.push(data[key]);
         }
         return regExp.test(tokens.join("|"));
     }
 }