KRB5: Remove spurious warning in logs
The option krb5_map_user is empty by default.
Therefore we should not confuse users wih warning
(Fri Nov 15 09:58:49 2016) [sssd[be[example.com]]] [parse_krb5_map_user]
(0x0200): Warning: krb5_map_user is empty!
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>