Fix ldapupdate.get_sub_dict() for missing named user
    
    The named user may not be present when ipa-server-dns and bind are not
    installed. NAMED_UID and NAMED_GID constants are only used with local
    DNS support.
    
    Fixes: https://pagure.io/freeipa/issue/8936
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Co-authored-by: François Cami <fcami@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>