Handle empty title attribute tooltips.

Source/core/rendering/HitTestResult.cpp

 /*
  * Copyright (C) 2006, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 214 matching lines @@
 }
 
 String HitTestResult::title(TextDirection& dir) const
 {
     dir = LTR;
     // Find the title in the nearest enclosing DOM node.
     // For <area> tags in image maps, walk the tree for the <area>, not the <img> using it.
     for (Node* titleNode = m_innerNode.get(); titleNode; titleNode = titleNode->parentNode()) {
         if (titleNode->isElementNode()) {
             String title = toElement(titleNode)->title();
-            if (!title.isEmpty()) {
+            if (!title.isNull()) {
                 if (RenderObject* renderer = titleNode->renderer())
                     dir = renderer->style()->direction();
                 return title;
             }
         }
     }
     return String();
 }
 
 const AtomicString& HitTestResult::altDisplayString() const
@@ skipping 231 matching lines @@
 {
     for (Node* node = m_innerNode.get(); node; node = NodeRenderingTraversal::parent(node)) {
         if (node->isElementNode())
             return toElement(node);
     }
 
     return 0;
 }
 
 } // namespace blink