fix(py/genkit/ai): add missing type annotation

[schlich]

py/packages/genkit/src/genkit/ai/_aio.py was missing the "or none" in its type annotation

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

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[ssbushi]

Thanks for the fix!