How to add a "continue shopping" button after emptying cart in Ubercart?
For this, you can override uc_empty_cart() function with theme_uc_empty_cart(); which returns your custom message on empty cart page.

You can get continuous shopping cart URL from Store > cart setting using variable_get('uc_continue_shopping_url', '').

Example:

  // Default function from uc_cart.module
  function theme_uc_empty_cart() {
    return '&lt;p&gt;'. t('There are no products in your shopping cart.') . '&lt;/p&gt;&lt;a href="'.variable_get('uc_continue_shopping_url', '').'" rel="continuous-shopping"&gt;continuous shopping&lt;/a&gt;';
  }