Eliminate dummy products array form element when no products in uc_order_edit_products_form()

In uc_order_edit_products_form(), if there are no products, then this form element is created:

  $form['products'][]['remove'] = array(
    '#value' => t('This order contains no products.'),
    '#cell_attributes' => array('colspan' => 'full'),
  );

This causes problems if you are altering this form and looking for element_children($form['products']). Having an array entry gives a false positive and causes code to execute when not intended.