Respect config.required for auth

[rainshen49]

I think the original intention of the author was to default to true if config.required isn't specified, but ||true will just always evaluate to true. This blocks me when I was following the onboarding flow of https://firebase.google.com/docs/genkit/get-started#gemini-google-ai

Checklist (if applicable):

• Tested (manually, unit tested, etc.) -- there were no previous unit tests for this bit.
• Changelog updated
• Docs updated