fix: handle undefined groups

tasks/main.yml

@@ -77,7 +77,7 @@
       name: nagios
       groups: www-data
       append: true
-  when: inventory_hostname in groups.owncloud_server | union(groups.docker_owncloud) | union(groups.docker_nextcloud)
+  when: inventory_hostname in groups.owncloud_server | default([]) | union(groups.docker_owncloud | default([])) | union(groups.docker_nextcloud | default([]))
 
 - name: Add cron job to update available packages every day
   ansible.builtin.cron: