InteractionWorkflow.py: getStateValueList return an empty dict instead of None.

product/ERP5Workflow/Document/InteractionWorkflow.py

@@ -404,7 +404,7 @@ class InteractionWorkflow(IdAsReferenceMixin("", "prefix"), Workflow):
     return 0
 
   def getStateValueList(self):
-    return None
+    return {}
 
   def showAsXML(self, root=None):
     if root is None: