fix(auth, android): allow nullable `accessToken` when creating `OAuthProvider`

[russellwheatley]

Description

OAuthProvider can have a null access token, this makes behaviour consistent across web, apple and android.

Related Issues

fixes https://github.com/firebase/flutterfire/issues/12791

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [ ] All existing and new tests are passing.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] The analyzer (melos run analyze) does not report any problems on my PR.
• [ ] I read and followed the Flutter Style Guide.
• [ ] I signed the CLA.
• [ ] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change.
• [ ] No, this is not a breaking change.

[meihdimtg]

I have a problem, registering a user with an email password and a custom token, except that this changed 1 month ago, no one can register on my application anymore, but on the web side it works.

However, the Firebase keys work correctly, the json files are in the root, I have an internal error, MISSING IDENTIFIER however the token returned by my api works for the NEXT JS version which has the same token as that of the app except that via the application and the flutter firebase_auth package this no longer works.