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

Which is very likely .Z > 0 releases.

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

bin/gen_release_notes.py

@@ -224,6 +224,8 @@ def get_features() -> typing.Generator[str, None, None]:
         with p.open('rt') as f:
             for line in f:
                 yield line
+    else:
+        yield "None"
 
 
 async def main() -> None: