Changed StackList.__init__

Aufgabe03a.py

-class Stack(list):
+class Stack():
+
     def __init__(self, l=[]):
         self.stack = l
 
@@ -35,7 +36,10 @@ class ListElement:
 
 class StackList:
 
-    def __init__(self, l):
+    def __init__(self, l=None):
+        if l is None:
+            self.first_element = None
+        else:
             self.first_element = ListElement(l[-1])
             element = self.first_element
             for value in l[1::-1]: