When a replica set is first initialized using rs.initiate(), it writes a note to that effect into the oplog as its first entry. However, this is written while the term is OpTime::kUninitializedTerm, and the entry therefore has no term field t:

{ "op" : "n", "ns" : "", "o" : { "msg" : "initiating set" }, "ts" : Timestamp(1563285225, 1), "v" : NumberLong(2), "wall" : ISODate("2019-07-16T13:53:45.914Z") }

This has been the case since 3.6 and remains so on current master.