cvsps: avoid comparison between None and a tuple in date sorting

Authored by durin42.

Description

cvsps: avoid comparison between None and a tuple in date sorting

Avoids badness on Python 3. I had to figure out which entries in this
object *could* be None experimentally, but I think I've got them all
now.

Differential Revision: https://phab.mercurial-scm.org/D3723

Details