IDL: Add support for [TreatNullAs=EmptyString]

Source/bindings/core/v8/custom/V8CustomXPathNSResolver.cpp

 // Copyright (c) 2008, 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 68 matching lines @@
     const int argc = 1;
     v8::Handle<v8::Value> argv[argc] = { v8String(m_isolate, prefix) };
     v8::Handle<v8::Function> function = lookupNamespaceURIFunc.IsEmpty() ? v8::Handle<v8::Function>::Cast(m_resolver) : lookupNamespaceURIFunc;
 
     v8::Handle<v8::Value> retval = ScriptController::callFunction(callingExecutionContext(m_isolate), function, m_resolver, argc, argv, m_isolate);
 
     // Eat exceptions from namespace resolver and return an empty string. This will most likely cause NamespaceError.
     if (tryCatch.HasCaught())
         return nullAtom;
 
-    TOSTRING_DEFAULT(V8StringResource<WithNullCheck>, returnString, retval, nullAtom);
+    TOSTRING_DEFAULT(V8StringResource<TreatNullAsNullString>, returnString, retval, nullAtom);
     return returnString;
 }
 
 void V8CustomXPathNSResolver::trace(Visitor* visitor)
 {
     XPathNSResolver::trace(visitor);
 }
 
 } // namespace WebCore