Fixed StackList.__str__

Aufgabe03a.py

@@ -50,6 +50,9 @@ class StackList:
                 element.next = ListElement(value)
 
     def __str__(self):
+        if self.isEmpty():
+            return "StackList()"
+
         return_string = ""
         element = self.first_element
         while element.next is not None: