Reviewed by Brady.

    Reviewed by Brady.

        Fixed semantic inaccuracy in String::operator NSString *(), where a NULL
        String became an empty NSString *, instead of nil. This will become important
        as we start using this conversion more.

        Added work-around for AppKit crash that semantic accuracy revealed.

        No testcase because I don't think there's a way to reflect this change
        in a webpage yet.

        * platform/PlatformString.h:
        (WebCore::String::operator NSString*):
        * platform/mac/TextFieldMac.mm:
        (-[NSSearchFieldCell _addStringToRecentSearches:]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@17378 268f45cc-cd09-0410-ab3c-d52691b4dbfc