add default exception to calculate method

Author: walpurr

app/src/main/java/hexlet/code/games/Calc.java

@@ -8,7 +8,7 @@ private static Integer calculate(int number1, int number2, char operator) {
             case '+' -> number1 + number2;
             case '-' -> number1 - number2;
             case '*' -> number1 * number2;
-            default -> null;
+            default -> throw new IllegalArgumentException(String.format("Unknown operator! Operator = %s", operator));
         };
     }