Followup for recent fix to 5079700, found by assertion added to Safari.

        * page/mac/FrameMac.mm:
        (WebCore::Frame::searchForLabelsBeforeElement):
        Fixed another place in this method where an empty NSString was being returned instead of nil.
        Cleaned up the style a little too.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20404 268f45cc-cd09-0410-ab3c-d52691b4dbfc