ItemRegister made new method to check if register is empty

src/main/java/no/ntnu/idatt1002/demo/data/Economics/ItemRegister.java

@@ -51,6 +51,15 @@ public class ItemRegister<T extends Item>{
         items.add(newItem);
     }
 
+    /**
+     * Check if items is empty.
+     *
+     * @return true or false depending on if items is empty.
+     */
+    public boolean isEmpty(){
+        return this.items.isEmpty();
+    }
+
     /**
      * Get the sum of all T´s=(Income or Expenses) in items.
      *