DNT should return 'null' if there is no client.

Source/modules/donottrack/NavigatorDoNotTrack.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * 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.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 50 matching lines @@
     return *supplement;
 }
 
 String NavigatorDoNotTrack::doNotTrack(Navigator& navigator)
 {
     return NavigatorDoNotTrack::from(navigator).doNotTrack();
 }
 
 String NavigatorDoNotTrack::doNotTrack()
 {
-    return frame() ? frame()->loader().client()->doNotTrackValue() : String();
+    if (!frame() || !frame()->loader().client())
+        return String();
+    return frame()->loader().client()->doNotTrackValue();
 }
 
 } // namespace WebCore