Correct the type signature of `account` in callbacks.

[w9]

☕️ Reasoning

The account argument can be undefined if it's not immediately after a sign-in. The current type signature does not reflect that.

🧢 Checklist

• [x] Documentation (Not applicable)
• [x] Tests
• [x] Ready to be merged

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Name	Status	Preview	Comments	Updated (UTC)
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