fix(type): correct the wrong type of the `getSigningKey` function arg…

[stegano]

Description

fix(type): correct the wrong type of the getSigningKey function argument

• If the err argument value is not null, the key argument value is undefined.

// Example
client.getSigningKey(header.kid, function (err, key) {
    console.log(`=== Test ===`);
    console.log(`err ≥`, err);
    console.log(`key ≥`, key);
    callback(err, key?.getPublicKey());
});

# Result
=== Test ===
err ≥ TypeError: jwks must be a JSON Web Key Set formatted object
key ≥ undefined

References

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• GitHub Issue/PR number addressed or fixed
• Auth0 Community post
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• Support forum thread
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Testing

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• [ ] This change adds test coverage for new/changed/fixed functionality

Checklist

• [ ] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [ ] All active GitHub checks for tests, formatting, and security are passing
• [ ] The correct base branch is being used, if not master