`/turnServer` doesn't specify error when homeserver has no TURN server

## Link to problem area:
https://spec.matrix.org/v1.10/client-server-api/#get_matrixclientv3voipturnserver
## Issue
In the spec, it does not tell homeservers what to respond with for requests to `/voip/turnServer` when there is no TURN server configured. Currently:
- Synapse and Dendrite return `200 OK` with an empty body.
- Conduit and Construct return empty strings for `username` and `password`, and an empty array for `uris`.
- Telodendria does not support this endpoint yet.

What would be the correct behavior? The empty body, an (`M_NOT_FOUND`) error code, or something else?

Start with the linked Matrix Client-Server API section for GET /_matrix/client/v3/voip/turnServer and compare the response behaviors listed for Synapse, Dendrite, Conduit, and Construct. Done means reaching and documenting an agreed response for homeservers without a configured TURN server.

Written by the indexing model from the issue text.