Better validation of auto suite-state polling tasks.

Automatic suite-state polling tasks are not allowed to define custom command-scripting (it is supplied by cylc) but this change this allows explicit blanking of inherited scripting: ```command scripting = # (nothing)```

Without this change use of any auto suite-state polling tasks means you can't put the Rose task-run command or similar under ```[[root]]```, as the inherited command scripting will fail validation when the polling tasks are checked.

@matthewrmshin - please review.