[firebase_auth] Fixes exceptions when signing in on iOS

[collinjackson]

Description

Avoids an exception when additionalProviderInfo is nil or has nil fields.

Related Issues

Fixes flutter/flutter#37133

This is already tested by our integration tests, but our test CI isn't failing on exceptions that kill the app on iOS, which is how this slipped through. Once that is fixed, test failures like this will prevent PRs from being merged. (flutter/flutter#37141, https://github.com/flutter/plugin_tools/pull/50)

For the moment we can confirm that the test failure is fixed by just looking at the test logs.

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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[kroikie]

LGTM