Handle null model fields when iterating

[sentry-io]

TypeError: 'NoneType' object is not iterable

Sentry Issue: STUDIO-FX6

TypeError: 'NoneType' object is not iterable
  File "celery/app/trace.py", line 734, in __protected_call__
    return self.run(*args, **kwargs)
  File "contentcuration/tasks.py", line 107, in getnodedetails_task
    return node.get_details()
  File "contentcuration/models.py", line 1713, in get_details
    "authors": list(filter(bool, node["authors"])),