[firebase_auth] fetchSignInMethodsForEmail returns null on iOS, empty array on Android

[timtraversy]

Description

As the issue I opened in the firebase sdk repo explains, a call to fetchProvidersForEmail()� in the native Firebase Auth SDKs for an unregistered user returns null on iOS, and an empty array on Android-so the plugin returns different results for the same line of Dart code depending on the platform.
The best option would be to standardize the native SDKs, but until that happens I'm proposing a slight change that returns an empty array on iOS if the SDK returns null.

This can't be tested without an integration test, and the firebase_auth library doesn't have any yet.

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[collinjackson]

This PR has been migrated to the new repo. See firebase/flutterfire#26