how can we update null value columns in whole table in sql server, without using column name?

Create Table Temp (Item Nvarchar(100), Col1 Nvarchar(10),Col2 Nvarchar(40))
Insert into Temp 
Select 'Item1',Null,'Test1' Union All
Select 'Item1',Null,Null Union All
Select 'Item1',Null,'Test1'Union All
Select 'Item1',Null,'Test1' Union All
Select 'Item1','Test1',Null

Declare @ColumnName Varchar(100),@Pos Int,@PrevPos Int=0,@UpdatedValue varchar(100),
        @Query varchar(1000),@TableName Varchar(40)

Select  @TableName= 'Temp',@UpdatedValue='''Test'''

While 1>0
Begin
    Set @ColumnName=''
    Select Top(1) @ColumnName = COLUMN_NAME,@Pos= ORDINAL_POSITION 
    From INFORMATION_SCHEMA.COLUMNS  
    Where TABLE_NAME=@TableName and IS_NULLABLE ='YES' and DATA_TYPE ='nvarchar'
    and ORDINAL_POSITION > @PrevPos

    If @ColumnName =''
       Break;
	
    Select @Query= 'Update ' + @TableName + ' Set '  + @ColumnName + ' = ' + 
    @UpdatedValue  +' Where ' + @ColumnName +' is null' , @PrevPos=@Pos 

    Exec(@Query)	
End

Select * from Temp 
Drop Table Temp

Item	Col1	Col2
-----  -----  -------
Item1	Test	Test1
Item1	Test	Test
Item1	Test	Test1
Item1	Test	Test1
Item1	Test1	Test