Stop Redis exception by not passing Redis an empty set to delete
Stop Redis exception by not passing Redis an empty set to delete

We use `redis_client.delete` to delete multiple keys at once, but this
raises a `redis.exceptions.ResponseError` if it is called with an empty list
or set. We should only call `redis_client.delete` when there is at least
one item to delete.

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