fix(auth, apple): fetchSignInMethodsForEmail if value is nil, pass empty array.

[russellwheatley]

Description

providers can be nil:https://github.com/firebase/flutterfire/blob/master/packages/firebase_auth/firebase_auth/ios/Classes/FLTFirebaseAuthPlugin.m#L1047

This PR check ifs nil and passes empty array.

The other issue is that the response seems to always come back as nil. I checked with a user that has email/password & google providers enabled and it still responded with nil. Appears to be a bug on the firebase-ios-sdk.

Related Issues

fixes #11293

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