Worklist.py: avoid changing None to tuple.

product/ERP5Workflow/Document/Worklist.py

@@ -180,7 +180,7 @@ class Worklist(IdAsReferenceMixin("worklist_", "prefix"), XMLObject):
         matches_ref_list = []
         if id == 'portal_type':
           v = self.getMatchedPortalTypeList()
-          matches = tuple(v)
+          if v: matches = tuple(v)
         elif id in ['validation_state', 'simulation_state']:
           if id == 'validation_state':
             matches_id_list = self.getMatchedValidationStateList()