title: How to create a 'null' default in Ansible
body: <p>I want 'lucy' to follow the user module creators' default behaviour which is to create and use a group matching the user name 'lucy'.  However for 'frank' I want the primary group to be an existing one; gid 1003.  So my hash looks like this:</p>
  
  <pre><code>lucy:
    comment: dog
  frank:
    comment: cat
    group: 1003
  </code></pre>
  
  <p>And my task looks like this:</p>
  
  <pre><code>- name: Set up local unix user accounts
    user:
      name: "{{ item.key }}"
      comment: "{{ item.value.comment }}"
      group: "{{ item.value.group | default(undef) }}"
    loop: "{{ users|dict2items }}"
  </code></pre>
  
  <p>This doesn't work, as undef is not recognised. Nor is anything else I can think of. 'null', 'None' etc. all fail. '' creates an empty string which is not right either. I can't find out how to do it.
  Any ideas?</p>