robust pop()

src/main/java/MyStack.java

+import java.util.EmptyStackException;
+
 public class MyStack {
 
     private Node top;
@@ -16,6 +18,7 @@ public class MyStack {
     }
 
     public Object pop() {
+        if (size() == 0) throw new EmptyStackException();
         Object payload = top.getPayload();
         top = top.getPrevious();
         return payload;