Code review comment for ~cgrabowski/maas:handle_different_channel_check_for_missing_channel

We should probably add another entry to VERSION_ISSUES for this specific case where a controller has no update_origin (and don't report the DIFFERENT_CHANNEL one in that case).

While the current message is misleading, I think it's correct to report lack of an update origin as an issue as that means the controller will never get updates (which has security-related implications).