erp5_advanced_ecommerce: default value to None

bt5/erp5_advanced_ecommerce/SkinTemplateItem/portal_skins/erp5_advanced_ecommerce/Product_getAdditionalService.zpt

 <tal:block tal:define="supply_line python: context.Product_getSaleSupplyLine();
                        additional_service python: supply_line.getAdditionalServiceValue();
-                       minimal_quantity python: getattr(supply_line, 'additional_service_min_quantity');
+                       minimal_quantity python: getattr(supply_line, 'additional_service_min_quantity', None);
                        ">
   <tal:block tal:condition="python: additional_service is not None">
     <label tal:content='python: additional_service.getDescription()'></label>