Fix GET /_matrix/federation/v1/user/devices/{userId} response

[SnackEngineer]

Don't send the fields master_key and self_signing_key when they are not defined for the queried user device.'

Before this change they would be sent and set to null in the JSON response object, which would violate the OpenAPI definitions (https://spec.matrix.org/v1.3/server-server-api/#get_matrixfederationv1userdevicesuserid).

12

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

Before this change they would be sent and set to null in the JSON response object, which would violate the OpenAPI definitions (https://spec.matrix.org/v1.3/server-server-api/#get_matrixfederationv1userdevicesuserid).

According to the spec, master_key and self_signing_key are not required fields.

The OpenAPI spec for the /user/devices/{userId} endpoint is here: https://github.com/matrix-org/matrix-spec/blob/7d2de48cb4774db60c63bc339daf089dd09387cd/data/api/server-server/user_devices.yaml#L20-L108

Where does the OpenAPI spec say that null isn't valid for an optional field?

What prior art in the Matrix spec are you leaning on that it's better to omit the fields?