Function get_json_optional 

pub async fn get_json_optional(
    client: &Client,
    url: &str,
    allowed: &[&str],
) -> Result<Option<Value>>

As get_json, but a 404 means absent rather than failed.

/.well-known/oauth-protected-resource legitimately does not exist on some hosts, and “absent” is a different answer from “the fetch went wrong”.