stack: prevent crash when topic is rooted on nullid
We exclude null from displayed base. This was causing a crash when trying to
display its description (we'll still have to handle the empty description case,
but it is now less urgent).
test: add topic on the first 2 changesets too
This will be used to add a test for topic rooted on nullid. We introduce this in
its own changeset to reduce the noise if the actual changeset.