dbwrap: Use tdb_null in db_ctdb_delete

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Christian Ambach <ambi@samba.org>

Autobuild-User(master): Christian Ambach <ambi@samba.org>
Autobuild-Date(master): Mon Mar 25 19:42:30 CET 2013 on sn-devel-104