fix(auth, web): use the device language when using setLanguageCode with null

[Lyokone]

Description

The inline documentation was currently wrong:
Android

  @Override
  public void setLanguageCode(
      @NonNull GeneratedAndroidFirebaseAuth.AuthPigeonFirebaseApp app,
      @Nullable String languageCode,
      @NonNull GeneratedAndroidFirebaseAuth.Result<String> result) {
    try {
      FirebaseAuth firebaseAuth = getAuthFromPigeon(app);

      if (languageCode == null) {
        firebaseAuth.useAppLanguage();
      } else {
        firebaseAuth.setLanguageCode(languageCode);
      }

      result.success(firebaseAuth.getLanguageCode());
    } catch (Exception e) {
      result.error(e);
    }
  }

iOS

- (void)setLanguageCodeApp:(nonnull AuthPigeonFirebaseApp *)app
              languageCode:(nullable NSString *)languageCode
                completion:
                    (nonnull void (^)(NSString *_Nullable, FlutterError *_Nullable))completion {
  FIRAuth *auth = [self getFIRAuthFromAppNameFromPigeon:app];

  if (languageCode != nil && ![languageCode isEqual:[NSNull null]]) {
    auth.languageCode = languageCode;
  } else {
    [auth useAppLanguage];
  }

  completion(auth.languageCode, nil);
}

The modifications in this PR align the behavior of web with Android and iOS.

Related Issues

closes #9458

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Breaking Change

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