avoid empty ul

An empty UL is not valid in HTML, but since we don't know if LIs are
added later on by JavaScript, we add an invisible LI by default now.

tpl/nav-meta-box.php

@@ -6,6 +6,7 @@ $tabs = \dokuwiki\template\sprintdoc\Template::getInstance()->getMetaBoxTabs();
 ?>
 <div class="tab-container">
     <ul class="meta-tabs">
+        <li class="a11y">&nbsp;</li>
         <?php
         foreach($tabs as $tab) {
             echo '<li>';