gistfile1.txt

---
- hosts: all
  gather_facts: False
  tasks:
    - shell: date
      when: false
      register: rvar
    - debug:
        msg: "Can I use an undefined var in a conditional?"
        when: rvar.rc==0
    - debug:
        msg: "Return code: {{rvar.rc}}"

gistfile2.txt

PLAY [all] ********************************************************************

TASK: [shell date] ************************************************************
skipping: [localhost]

TASK: [debug ] ****************************************************************
ok: [localhost] => {
    "msg": "Can I use an undefined var in a conditional?"
}

TASK: [debug ] ****************************************************************
fatal: [localhost] => One or more undefined variables: 'dict object' has no attribute 'rc'

FATAL: all hosts have already failed -- aborting

PLAY RECAP ********************************************************************
           to retry, use: --limit @/home/hugh/test.retry

localhost                  : ok=1    changed=0    unreachable=1    failed=0