From the ansible docs: If a required variable has not been set, you can skip or fail using Jinja2’s defined test. For example:
tasks:
- shell: echo "I've got '{{ foo }}' and am not afraid to use it!"
when: foo is defined
- fail: msg="Bailing out. this play requires 'bar'"
when: bar is not defined
So in your case, when: deployed_revision is not defined
should work