Allow broadcasts which have no created_at to be compared
Allow broadcasts which have no created_at to be compared

This adds to the `__le__` method on the `BroadcastMessage` class to
allow two BroadcastMessages with no `updated_at` to be compared.
Previously, the method expected at least one message to have
`updated_at` set, but this is not necessarily the case.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin