- fix <rdar://problem/4782422> REGRESSION: -[stringByEvaluatingJavaScriptFromString]
          returns an empty string instead of nil for non-string results

        * page/mac/WebCoreFrameBridge.mm:
        (-[WebCoreFrameBridge stringByEvaluatingJavaScriptFromString:forceUserGesture:]):
        Added a call to nsStringNilIfEmpty to restore the old behavior. It might be nice to
        distinguish the empty string from a non-string result, but there's no obvious way
        to do that, and for compatibility it's probably best to just restore the old
        behavior instead.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20999 268f45cc-cd09-0410-ab3c-d52691b4dbfc