Sql case statement

How would I formulate a case statement in this select,  which would move in 'N/A'  if the "Contract.ContractId" is = spaces.
It is a varchar(25) column. I need to include the check for isnull on this column.


SELECT DISTINCT
isnull(Contract.ContractId,'N/A') as [FormerContractID],
isnull (Contract.ContractOid, 0) as [FormerContractOid]

FROM            Contract

left JOIN ContractItem ON
      Contract.ContractOid = ContractItem.ContractOid AND ContractItem.IsTerminated = 1
      
left join
    ContractEquipment ON ContractEquipment.ContractEquipmentOid = ContractItem.ContractEquipmentOid

order by [FormerContractId]