Allow specifying STUN server (without TURN servers)

This issue has been migrated from [#15581](https://github.com/matrix-org/synapse/issues/15581).

---

**Description:**

I want to use a public `STUN` server, such as `stun.nextcloud.com:443` or `stun.l.google.com:19302` respectively. So following https://github.com/matrix-org/synapse/blob/def480442d752f1951cf7f790be873489a09c432/docs/turn-howto.md#synapse-setup I set my `turn_uris` to `["stun:stun.nextcloud.com:443"]` and restarted Synapse.

**Expected outcome:**

`_matrix/client/r0/voip/turnServer`
```json
{
"uris": [
"stun:stun.nextcloud.com:443"
]
}
```

**Actual outcome:**

`_matrix/client/r0/voip/turnServer` returns an empty JSON `{}`

---

Only when supplying all other configuration options (`turn_shared_secret`, `turn_user_lifetime`, `turn_allow_guests`) with a value as well, Synapse starts giving back the corresponding URIs when calling `_matrix/client/r0/voip/turnServer`.

I'm actually unsure if it's working at all then, but making use of `STUN` should be possible without using `TURN`, right?

Start with the _matrix/client/r0/voip/turnServer endpoint and the turn_uris configuration described in docs/turn-howto.md. Reproduce the response using only a STUN URI, then trace how the endpoint handles configurations without TURN options. Done means the endpoint returns the configured STUN URI without requiring unrelated TURN settings.

Written by the indexing model from the issue text.