An upcoming commit will change the type of index entries from tuple
to something else. This type may not be initialized/available at
module init time.
We prepare for this future change by making null entries a per-instance
variable on the index type.
In theory, this does add some overhead (a new 8-tuple per index
instance). However, the overhead should be negligible compared to
the run-time overhead of creating an index entry for each revision
in the index.