fix(auth,web): convert Auth errors that omit customData without throwing

[SelaseKay]

Description

On Flutter web, some Firebase Auth JS errors omit customData. getFirebaseAuthException cast that missing value to AuthErrorCustomData, which threw a TypeError after the app had already handled the sign-in failure.

This treats customData as nullable and still maps code / message (and email/phone/tenant when present) into FirebaseAuthException.

Related Issues

• Fixes [firebase_auth_web] Uncaught TypeError after showing INVALID_LOGIN_CREDENTIALS error (web_utils.dart getFirebaseAuthException) #18680

Checklist

• 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.

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