I have found that the vesion of tpl_modules_product_listing.php in the mod folder (and in the zip) does not include the last tweak I made to my test site version. This would account for blank list items; replacing this section in your copy of the file will correct the issue.
PHP Code:
<?php
/**
 * load the list_box_content template to display the products
 */
if ($product_listing_layout_style == 'columns') {
  if (
PRODUCT_LISTING_GRID_SORT) { 
    echo 
'<ul id="gridSorter">' PRODUCT_LISTING_GRID_SORT_TEXT;
    for (
$col=0;$col<sizeof($grid_sort);$col++) {
      if (
$grid_sort[$col]['text']) echo '<li class="item">' $grid_sort[$col]['text'] . '</li>';
    }
    echo 
'</ul>';
  }
This line:
if ($grid_sort[$col]['text']) echo '<li class="item">' . $grid_sort[$col]['text'] . '</li>';
is what is changed to eliminate blank items.

I will update the mod in Plugins. I apologize for the inconvenience.