test.yaml

- name: derp
  gather_facts: false
  hosts: all
  vars:
    foo: bar
    missing: derp

  tasks:
    - command: echo yes
      when: foo.startswith('nope') or (missing is defined and missing.starts_with('derp'))
      
$ ansible-playbook -i derp, test.yaml -vv -c local

PLAY [derp] *******************************************************************

TASK: [command echo yes] ******************************************************
skipping: [derp]