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 #12791

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[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.