add methods to decrement occupied spots and check if the parking lot is empty

code/CarSpotController.cpp

@@ -29,7 +29,15 @@ class CarSpotController {
             this->occupied_spots++;
         }
 
+        void decrement_occupied_spots () {
+            this->occupied_spots--;
+        }
+
         bool is_full() {
             return this->occupied_spots == this->total_spots;
         }
+
+        bool is_empty() {
+            return this->occupied_spots == 0;
+        }
 };