Add test for empty entity.


git-svn-id: https://gnunet.org/svn/libmicrohttpd@33174 140774ce-b5e7-0310-ab8b-a85725594a96
-fix #3397

git-svn-id: https://gnunet.org/svn/libmicrohttpd@33266 140774ce-b5e7-0310-ab8b-a85725594a96
never notify twice

git-svn-id: https://gnunet.org/svn/libmicrohttpd@33312 140774ce-b5e7-0310-ab8b-a85725594a96