[devtools] Return empty array from DOMNode.children() rather than null.

Source/devtools/front_end/DOMAgent.js

 /*
  * Copyright (C) 2009, 2010 Google Inc. All rights reserved.
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 334 matching lines @@
                 callback(this.children());
             return;
         }
 
         /**
          * @this {WebInspector.DOMNode}
          * @param {?Protocol.Error} error
          */
         function mycallback(error)
         {
+            if (!this._children) {
+                // By this moment children should have been pushed in case there were any.
+                // Fall back for nodes with no children.
+                this._children = [];
+            }
             if (callback)
                 callback(error ? null : this.children());
         }
 
         DOMAgent.requestChildNodes(this.id, undefined, mycallback.bind(this));
     },
 
     /**
      * @param {number} depth
      * @param {function(Array.<WebInspector.DOMNode>)=} callback
@@ skipping 1370 matching lines @@
     setInspectModeEnabled: function(enabled, inspectShadowDOM, config, callback)
     {
         DOMAgent.setInspectModeEnabled(enabled, inspectShadowDOM, config, callback);
     }
 }
 
 /**
  * @type {?WebInspector.DOMAgent}
  */
 WebInspector.domAgent = null;