erp5_commerce: shopping cart can be None

bt5/erp5_commerce/SkinTemplateItem/portal_skins/erp5_commerce/SaleOrder_getShoppingCartItemList.py

@@ -2,7 +2,10 @@
   Return list of shopping cart items.
   XXX : get order_line as tree not just on first level
 """
-shopping_cart_order_lines = context.SaleOrder_getShoppingCart().contentValues()
+shopping_cart = context.SaleOrder_getShoppingCart()
+if not shopping_cart:
+  return []
+shopping_cart_order_lines = shopping_cart.contentValues()
 if include_shipping:
   return shopping_cart_order_lines
 else: