This has been a problem with Element Web, as it will proble /register
with an empty body, which gave this error:

```
curl -d '{}' -HContent-Type:application/json /_matrix/client/v3/register

{"errcode": "M_UNKNOWN",
 "error": "Invalid username"}
```

And Element Web would choke on it. This changes that so we reply
instead:

```
{"errcode": "M_FORBIDDEN",
 "error": "Registration has been disabled. Only m.login.application_service registrations are allowed."}
```

Also adds a test for this.

See https://github.com/element-hq/element-web/issues/27993



---------

Co-authored-by: Andrew Morgan <andrew@amorgan.xyz>