pkinit: don't fail when no pkinit servers found
    
    If we issue pkinit-status after an upgrade from a pre-4.5 ipa
    version, it would have failed with KeyError since the
    pkinit_server_server of IPA config was never initialized.
    
    https://pagure.io/freeipa/issue/7144
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>