matrix-docker-ansible-deploy/roles/matrix-sygnal/tasks/validate_config.yml

- name: Fail if no Sygnal apps defined
  fail:
    msg: >-
      Enabling Sygnal requires that you specify at least one app in `matrix_sygnal_apps`      
  when: "matrix_sygnal_enabled and matrix_sygnal_apps|length == 0"