I don't really want to argue about http theoretical (working code is better than http status code arguments), but .... 400 (Bad Request) seems like a weird response to signify "no subscribers".  The w3c say it means "The request could not be understood by the server due to malformed syntax", which is arguably not what has happened - we've made a valid request, but the result was that there was no-one to notify.  I'd argue this should just be a 200, with a payload of how many folk were notified, or, at a push, a 404 (if we squint at it you could read no subscribers as a missing "people to notify" resource).