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):
1a284014d2/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.
