Don't fail if config-show does not return servers
When uninstalling a cluster and only hidden servers are left,
config-show can return a result set without ipa_master_server entry.
Fixes: https://pagure.io/freeipa/issue/7892
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Thomas Woerner <twoerner@redhat.com>
Reviewed-By: Francois Cami <fcami@redhat.com>