Support new broadcasts (without area IDs)

Previously we relied on the API defaulting this field to an empty
array [1], but that conflicts with using it to decide whether a
broadcast is custom or created in this app.

[1]: https://github.com/alphagov/notifications-api/blob/3779146cc51385327cef90390af1f62db983919f/app/models.py#L2342

app/models/broadcast_message.py

@@ -111,7 +111,7 @@ class BroadcastMessage(JSONModel):
 
     @property
     def area_ids(self):
-        return self._dict['areas']['ids']
+        return self._dict['areas'].get('ids', [])
 
     @area_ids.setter
     def area_ids(self, value):