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