CSS question: No border around empty sidebar

When the sidebar is empty, there appears empty sidebar's border at the bottom of the page (see attached image)
This is the site url: clean2.lafsdesign.com/blog/

When I tried hiding this, it also removed border for sidebar (with widgets inside).
How can I remove the border for empty sidebar while keeping the border when there is an widget inside?
I'd really appreciate your help,
Thank you in advance,