Stop defaulting "ids" to an empty array

This is so we can distinguish custom broadcasts in the Admin app
[1]. I've also extended the POST test for custom broadcasts to
check we're correctly reading data for "names", as this wasn't
being tested previously.

[1]: 411fda81c0

tests/app/broadcast_message/test_rest.py

@@ -160,7 +160,6 @@ def test_create_broadcast_message(admin_request, sample_broadcast_service, train
     assert response['created_at'] is not None
     assert response['created_by_id'] == str(t.created_by_id)
     assert response['personalisation'] == {}
-    assert response['areas']['ids'] == []
     assert response['areas']['simple_polygons'] == []
     assert response['content'] == 'Some content\n€ŷŵ~\n\'\'""---'