a get method should not modify the object

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@16979 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/XMLMatrix.py

@@ -474,7 +474,7 @@ class XMLMatrix(Folder):
           Returns the cell range as a list of index ids
       """
       if not hasattr(self, 'index'):
-        self.index = PersistentMapping()
+        return []
       cell_range = self.index.get(base_id, None)
       if cell_range is None: return None
       result = []