Worklist.py: causality state getter returns an empty string instead of None...

Worklist.py: causality state getter returns an empty string instead of None when it is not definded.

product/ERP5Workflow/Document/Worklist.py

@@ -133,6 +133,7 @@ class Worklist(IdAsReferenceMixin("worklist_", "prefix"), XMLObject):
         """ return value of matched keys"""
         #self.var_matches = {}
         matches = None
+        matches_ref_list = []
         if id == 'portal_type':
           v = ''.join(self.matched_portal_type)
           if tales_re.match(v).group(1):
@@ -142,19 +143,18 @@ class Worklist(IdAsReferenceMixin("worklist_", "prefix"), XMLObject):
             matches = tuple(v)
         elif id == 'validation_state':
           matches_id_list = self.getMatchedValidationStateList()
-          matches_ref_list = []
           for state_id in matches_id_list:
             matches_ref_list.append(self.getParent()._getOb(state_id).getReference())
           matches = tuple(matches_ref_list)
         elif id == 'simulation_state':
           matches_id_list = self.getMatchedSimulationStateList()
-          matches_ref_list = []
           for state_id in matches_id_list:
             matches_ref_list.append(self.getParent()._getOb(state_id).getReference())
           matches = tuple(matches_ref_list)
         elif id == 'causality_state':
           matches_id = self.getMatchedCausalityState()
-          matches_ref_list = []
+          if matches_id is None:
+            matches_id = ''
           matches_ref_list.append(matches_id)
           matches = tuple(matches_ref_list)
         else: