return empty list for empty input.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@33515 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/MirrorMovementGroup.py

@@ -51,6 +51,8 @@ class MirrorMovementGroup(MovementGroup):
   def _separate(self, movement_list):
     # record if mirrored or not in simulation movements.
     mapping_dict = {}
+    if len(movement_list) == 0:
+      return []
     for movement in movement_list:
       if _isMirrored(movement):
         applied_rule = movement.getParentValue()