Instead of being "padding," it appears to be an anchor tag, most likely intended for holding an icon in drop-down menus.
If possible, remove the empty anchor tag in that spot.
<li aria-haspopup="true" class="folder-collection folder">
<a href="#" onclick="return false;"></a> <!-- < this anchor is the space -->
<a href="http://www.roboticsrookie.com/reviews/">Reviews</a>
<span class="delimiter">/</span>
<div class="subnav">
<ul>
<li class="page-collection"><a href="/robotics-reviews/">Robotics Reviews</a></li>
<li class="page-collection"><a href="/book-reviews/">Book Reviews</a></li>
<li class="blog-collection"><a href="/reviews/">Reviews</a></li>
</ul>
</div>
</li>
Alternatively, you can add CSS:
li.folder-collection.folder > a:nth-child(1) { display: none;}
This action should eliminate the first empty anchor.
Your CSS selector was incorrect. #main-navigation a.folder
searches for an anchor with the class "folder." In essence,
<a class="folder" href="#"></a>
.
What you actually have is a list item with the class folder and an anchor tag inside that list item. Or
<li class="folder-collection folder"><a href="#"></a></li>
.
You can't easily target that particular anchor because there's nothing distinguishing about it. It lacks a unique class or id attribute compared to other anchors within the list item. Therefore, consider using more precise selectors like nth-child to single it out.