Fix: TypeError: 'NoneType' object is not iterable

```
Traceback (most recent call last):
  File "./bin/ansible-lint", line 68, in <module>
    sys.exit(main(sys.argv[1:]))
  File "./bin/ansible-lint", line 56, in main
    playbooks |= set(utils.find_children(arg))
  File "~/Dev/ansible-lint/lib/ansiblelint/utils.py", line 54, in find_children
    items = _playbook_items(pb_data)
  File "~/Dev/ansible-lint/lib/ansiblelint/utils.py", line 46, in _playbook_items
    return [item for play in pb_data for item in play.items()]
TypeError: 'NoneType' object is not iterable
```