Add none-type checking to get_all_objects/actions methods in Bundle

[ikiril01]

We need to add None-type checking to some of the methods in the Bundle that return lists of all of the Actions/Objects, otherwise we'll get errors such as

    all_objects = self.get_all_objects(include_actions=True)
  File "python-maec\maec\bundle\bundle.py", line 477, in get_all_objects
    for related_obj in associated_object.related_objects:
TypeError: 'NoneType' object is not iterable