GREEN: Fix empty string bug

- Changed return value from 1 to 0 for empty string
- Test now passes: empty string returns 0

lib/string_calculator.dart

@@ -12,7 +12,7 @@ class StringCalculator {
   int add(String numbers) {
     // Bug 1: Empty string handling
     if (numbers.isEmpty) {
-      return 1; // Should return 0
+      return 0; // Fixed: Return 0 for empty string
     }
 
     // Bug 2: Single number parsing