We can use the ISNULL() function in SQL Server. The given below code-sample might help you.
SELECT SUSR.ID
,SUSR.EmailID
,SUSR.MobileNo
,ISNULL(SUSR.NickName, SUSR.FirstName +' '+ SUSR.LastName) as SubrName
,SUSR.StatusID
FROM [SubscriberUser] SUSR
order by SUSR.ID desc