According to https://www.zabbix.com/documentation/4.0/manual/config/items/item#configuration item unit "s" is translated to “yyy mmm ddd hhh mmm sss ms”. But if the item value is "0" no unit is displayed.
This behaviour might be intended but it is misleading because normally you assume that there must be an error in the item configuration. Additionally this behaviour is not consistent with other units which are displayed even in this case.
So I think that there should be always displayed an appropriate unit.