How to create an empty file with Ansible?

The documentation of the file module says

If state=file, the file will NOT be created if it does not exist, see the copy or template module if you want that behavior.

So we use the copy module, using force=no to create a new empty file only when the file does not yet exist (if the file exists, its content is preserved).

- name: ensure file exists
  copy:
    content: ""
    dest: /etc/nologin
    force: no
    group: sys
    owner: root
    mode: 0555

This is a declarative and elegant solution.