How to update all varchar, nvarchar or char field of a SQL Server database and set empty string in the place of NULL?

declare @query varchar(MAX)
SET @query='';
select @query=@query+' UPDATE '+table_name+' SET '+column_name+' = isnull('+column_name+','''');'
from (
    SELECT table_name=sysobjects.name,
             column_name=syscolumns.name
        FROM sysobjects 
        JOIN syscolumns ON sysobjects.id = syscolumns.id
        JOIN systypes ON syscolumns.xtype=systypes.xtype
       WHERE sysobjects.xtype='U' and (systypes.name='nvarchar' or systypes.name='varchar' or systypes.name='char')
) as a

exec (@query)