how to solve isnull () in SQL

-----------------heere we want Updated By Name

select  c.ID as CUSTOMERLOANDATAID,c.FIRSTNAME,c.LASTNAME,
 isnull(UU.FIRSTNAME + ' '+ UU.LASTNAME,'') as  UpdatedByName
 from CUSTOMERLOANDATA c with(nolock)
Left Outer join USERMASTER UU (nolock) on UU.ID = C.UpdatedBy
where FILELOGID=(select top(1) filelogid  from CUSTOMERLOANDATA with(nolock) where
 QCUSERID = 13
ORDER BY FILELOGID)
and
 QCUSERID = 13
 ORDER BY FILELOGID, RECORDNO