NSS: Handle ENOENT when doing initgroups by UPN
https://fedorahosted.org/sssd/ticket/2598
We need to return an empty result in cases an initgroups lookup by UPN
doesn't return anything. Please note testing with "id user" is not
sufficient as id calls a getpwnam first.
Reviewed-by: Pavel Reichl <preichl@redhat.com>