RED: Test exposing empty string bug

- Test expects empty string to return 0
- Currently returns 1 instead
- Expected: <0>, Actual: <1>

test/string_calculator_test.dart

@@ -9,6 +9,8 @@ void main() {
       calculator = StringCalculator();
     });
 
-    // Tests will be added as we hunt bugs
+    test('empty string returns 0', () {
+      expect(calculator.add(''), equals(0));
+    });
   });
 }