Null instances for SegmentId and RecordId

In many cases in the past I would have had the need for null instances of SegmentId and RecordId. For OAK-5790 it will likely again simplify the implementation.

So far this was hard to achieve as SegmentId instances carry so much state along with themselves. However, with the refactoring done in OAK-3690 I think this is now finally doable.