validation error when primary/secondary links empty

After searching all over these forums for an answer to this issue, I thought I'd raise it here.

Using the same primary/secondary link code in my customized page.tpl.php as in the stock box_grey distribution, the <ul></ul> tags are always printed, regardless of whether the "show primary/secondary links" checkboxes on the theme settings page are marked or not. As the link boxes are empty, the <ul> tags do not contain any <li> elements, resulting in an xhtml validation error.

  <?php if (is_array($primary_links)) : ?>
    <ul id="primary">
    <?php foreach ($primary_links as $link): ?>
      <li><?php print $link?></li>
    <?php endforeach; ?>
    </ul>
  <?php endif; ?>

  <?php if (is_array($secondary_links)) : ?>
    <ul id="secondary">
    <?php foreach ($secondary_links as $link): ?>
      <li><?php print $link?></li>
    <?php endforeach; ?>
    </ul>
  <?php endif; ?>

results in:

<ul id="primary">
</ul>
  
<ul id="secondary">
</ul>