make empty string returnable by ReturnValue

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/16621004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15054 ce2b1a6d-e550-0410-aec6-3dcde31c8c00