Skip empty branch/status from release-cycle file

[m-aciek]

Empty strings in 3.16 metadata https://peps.python.org/api/release-cycle.json break the script after parsing.

Related to python/peps@f29099e commit.

[m-aciek]

We could have a weekly schedule of the test workflow to catch regressions like this?

[hugovk]

This was caught very quickly (an hour after merge) via a Sentry alert on the docs build:

python/peps#4829 (review)