AuthenticationData requires type but fallback forbids using type

[krille-chan]

Link to problem area: https://spec.matrix.org/unstable/client-server-api/#client-behaviour-1-1

Issue
If I want to use the delete_device endpoint:
https://spec.matrix.org/unstable/client-server-api/#post_matrixclientr0delete_devices

I first send this without the auth object in the JSON and then, when using SSO, I would like to use the fallback for authentication:
https://spec.matrix.org/unstable/client-server-api/#sso-during-user-interactive-authentication

But the spec says here:

Once the flow has completed, the client retries the request with the session only.

and indeed Element web is sending:

{
  "auth": {
    "session": "xxxxx"
  },
  "devices": [
    "QBUAZIFURK",
    "AUIECTSRND"
  ]
}

and this works but in the documentation of the delete_device endpoint we have:

type | string | Required: The login type that the client is attempting to complete.

Expected behaviour
Making the type no longer required may fix this but would lead to that it would be allowed send empty JSON objects in auth and I dont know if this would be a good idea.