In _layouts/post.html, it looks like there's something wrong with the loop logic here. Rather than try to explain, I'll just show you. This is from your example site. I'm afraid I'm too new to this syntax to know what's going on - but I suspect there's an empty data element inside site.categories
The generated HTML (cleaned up whitespace only) - note the first list element is visually blank.
<div class="panel-body">
<ul>
<li><a href="/jekyllBasicBlog"></a></li>
<li><a href="/jekyllBasicBlog/blog/post1">Post1</a></li>
</ul>
</div>