Copilot: handle "ok" status message when no user is set (#8116)

In #6954 a user has trouble using copilot. We haven't gotten to the
bottom of the problem, but one problem is that apparently sometimes (I'm
going to find out when) copilot sends an `"OK"` status message without a
username. This is from the user's logs:

2024-02-20T15:28:41-03:00 [ERROR] failed to deserialize response from
language server: missing field `user`. Response from language server:
"{\"status\":\"OK\"}"

The official `copilot.vim` plugin handles this as if the user is not
authenticated (!= authorized):


https://github.com/github/copilot.vim/blob/1a284014d2e0baf367706a94b2a9ee5fd56fd457/autoload/copilot.vim#L574-L579

So that's what I'm doing here too.

Release Notes:

- Fixed wrong handling of Copilot sign-in status in rare cases.