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]: https://github.com/alphagov/notifications-admin/pull/4014/commits/411fda81c0ff2d87c5553b1c5eb50c6023078797

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\'\'""---'
 

tests/app/v2/broadcast/test_post_broadcast.py

@@ -107,6 +107,8 @@ def test_valid_post_cap_xml_broadcast_returns_201(
     assert len(response_json['areas']['simple_polygons'][0]) == 23
     assert response_json['areas']['simple_polygons'][0][0] == [53.10562, 0.244127]
     assert response_json['areas']['simple_polygons'][0][-1] == [53.10562, 0.244127]
+    assert response_json['areas']['names'] == ['River Steeping in Wainfleet All Saints']
+    assert 'ids' not in response_json['areas']  # only for broadcasts created in Admin
 
     assert response_json['starts_at'] is None
     assert response_json['status'] == 'pending-approval'