histedit: fix serializing of None backupfile
If the histedit backupfile was None (like if evolve is enabled) it would get
serialized as 'None' into the state file. Later if the histedit was aborted and
the top most commit was unreachable (ex: if it was obsolete or stripped),
histedit would try to unbundle the backupfile and try to read .hg/None.
This fixes it to not serialize None. Since it only happens with evolve, I'm not
sure how to add test coverage here.