SDAP: Don't log an op failure when no users are found
    
    When no users are found during the search users process, just log (at
    TRACEL_ALL level) that zero users were retrieve and avoid logging (at
    OP_FAILURE level) that a failure has occurred, which may end up
    misleading admins, giving them the impression that something wrong has
    happened.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3089
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>