StringImpl::copy() should not ever return per-thread empty string, because this function's result is for using on other threads. This was broken in <http://trac.webkit.org/changeset/41917> - StringImpl::create() has a special case for empty strings, so calling it is not appropriate.
Created attachment 31663 [details] proposed fix
Committed revision 44952.
See <rdar://6964477> and its duplicates.