如何在产品列表页面的排序中最后设置空值产品?

Hello i have main 4 product like

     product_name  attribute_name attribute_value
----------------------------------------------------------
     test1           catname        abc
     test2           catname        ccc
     test3           catname        ###
     test4           catname        bbb

when i am trying sort products by catname on product listing page than i am getting null value product test3 first , but i need it in last. i have to ignore null attribute value.

I have try in toolbar.phtml for asc and desc but its not which i want.

Can anyone help me??

You can try as

 ORDER BY if(attribute_value = '' or attribute_value is null,1,0),attribute_value ,product_name