bin/gen_release_notes.py: Return "None" if there are no new features

Which is very likely .Z > 0 releases.

Fixes: 86079447
       ("scripts: Add a gen_release_notes.py script")
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit c6d41e7f)