Squash Authorship should not add an empty Authorship log when no authorship logs in squashed commits

[acunniffe]

Current behavior: If an engineer makes 5 commits (without git ai installed), CI command and the bot will add an empty authorship log.

Desired behavior:

• If there are no authorship logs for the 5 commits being squashed, do not write an empty one
• If there are any authorship logs (even blank ones indicating all-human), write an empty one
• If there are authorship logs with AI attributions, write one when squashing