I can’t find the store/order section of your site to do a test run of how the widget behaves, but if the plugin is really hiding its widget when the cart is empty (not even the widget title or a simple placeholder “nothing in cart” text are left), then I assume you’re seeing an empty sidebar.
The theme’s sidebar(s) visibility is controlled by the theme settings and it is not dependent on the presence (or lack) of widgets in the sidebar.