The discussion_id method can return `nil` if a previously cached value
does not exist. This normally does not happen because we process notes
from GitHub in ID order, so the note we're replying to should already be
processed.

But we've seen production data where the discussion_id is nil. This may
be a GitHub API problem where notes are missing or returned out of order,
or a problem with our cache storage.

This falls back to generating a new discussion_id when this edge
case happens.

Changelog: fixed