playbook

---
# file: test.yml
- hosts: localhost
  vars:
    foodata: null
  roles:
    - { role: foo, when: foodata }
---
# file: roles/foo/tasks/main.yml

- debug: msg="task1"
- debug: msg="task2"
  with_subelements:
    - foodata
    - barsub
- debug: msg="task3"

run

% ansible-playbook -i hosts test.yml

PLAY [localhost] **************************************************************

GATHERING FACTS ***************************************************************
ok: [localhost]

TASK: [foo | debug msg="task1"] ***********************************************
skipping: [localhost]

TASK: [foo | debug msg="task2"] ***********************************************
fatal: [localhost] => subelements lookup expects a dictionary, got 'None'

FATAL: all hosts have already failed -- aborting

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

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