sysdb: sysdb_get_certmap() allow empty certmap
Since sysdb_get_certmap() returns the user name hint information as well
it should return a result even if there are no certmaps.
Related to https://pagure.io/SSSD/sssd/issue/3395
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>