how can remove null values

DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)
select @cols = STUFF((SELECT distinct ',' + QUOTENAME(StuDocname)
                    from StudentDocuments
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)')
        ,1,1,'')

set @query = 'SELECT StudentCode,Stuname, ' + @cols + ' from
             (
                select DISTINCT SC.StuCurrentId,StudentCode,upper(First_Name + '' '' + Middle_Name + '' '' + Last_Name) as Stuname
                ,Sd.StuDocname,SASD.IsSelected,(case when IsSelected = 1 then ''YES''
                 when IsSelected = 0 then ''NO''
            end) as status
 from tbl_Student_Current SC
inner join StudentAdmission SA on SC.StudentId=SA.StudentId
inner join StudentAdmission_StudentDocuments SASD on SA.StudentId=SASD.StudentId
left outer join StudentDocuments SD on SASD.StudentDocId=SD.StuDocId

            ) x
            pivot
            (
                MAX(status)
                for StuDocname in (' + @cols + ')
            ) p '

execute(@query)