As a result of the Remove Stable Optime Candidates List (PM-1713) project, it is possible to have a case where there are no oplog entries before the oplog truncate after point (computer from the all-durable timestamp, which is at or after the stable optime candidate).  This happens when an oplog hole is open long enough that the size of the oplog entries after the hole is bigger than the configured oplog size, and so all entries prior to the stable timestamp get truncated

We cannot handle this case; we need an oplog entry before the truncate point to know when to start fetching. So we must ensure during truncation that we leave a record at or before the truncate point.