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

Source/devtools/front_end/DOMAgent.js

Index: Source/devtools/front_end/DOMAgent.js
diff --git a/Source/devtools/front_end/DOMAgent.js b/Source/devtools/front_end/DOMAgent.js
index 45467bc785d2b124fa86413c1df121a4feaaf940..4e3d2bbb83a4126eb143c79f799132d2d8ef0172 100644
--- a/Source/devtools/front_end/DOMAgent.js
+++ b/Source/devtools/front_end/DOMAgent.js
@@ -352,6 +352,11 @@ WebInspector.DOMNode.prototype = {
          */
         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());
         }