Change UString to use a 0 rep for null strings instead of a null object.

Reviewed by Oliver Hunt.

No performance impact.

* JavaScriptCore.exp:
* runtime/InternalFunction.cpp:
(JSC::InternalFunction::InternalFunction):
* runtime/JSString.h:
(JSC::RopeBuilder::JSString):
* runtime/UString.cpp:
(JSC::initializeUString):
* runtime/UString.h:
(JSC::UString::UString):
(JSC::UString::data):
(JSC::UString::size):
(JSC::UString::isNull):
(JSC::UString::isEmpty):
(JSC::UString::cost):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@57055 268f45cc-cd09-0410-ab3c-d52691b4dbfc