The "STALE" conclusion has shipped this January and is basically a
conclusion that can only be reported by GitHub and not explicitly set by
any integrations.
Fixes `panic: unsupported status: ""`
This occurs when a CheckRun has status "IN_PROGRESS" (or any other than
"COMPLETED") and when its `conclusion` would be null. I previously
didn't account for this.
This adds support for parsing state of an in-progress CheckRun.