index: don't include nullid in the internal "length" field
This is a follow-up to 781b2720d2ac (index: don't include nullid in
len(), 2018-07-20). I intentionally didn't update the "index stats",
so it will also not include the space for nullid after this patch.
Differential Revision: https://phab.mercurial-scm.org/D4162