test(auth): Fix e2e test (#2880)

`additionalInfo` now returns and empty map instead of `null` by default

Author: dnys1

packages/auth/amplify_auth_cognito/example/integration_test/sign_in_test.dart

@@ -91,7 +91,7 @@ void main() {
             username: username,
             password: password,
           );
-          expect(result.nextStep.additionalInfo, isNull);
+          expect(result.nextStep.additionalInfo, isEmpty);
         });
 
         asyncTest(