Don't even display an empty h5.top-term if largo_top_term() returns ''

[benlk]

After #1090 for #1082, many empty top terms will still output the following code:

<h5 class="top-tag"></h5>

This is because we typically create the top tag with a variant on the following markup:

<h5 class="top-tag"><?php echo largo_top_term(); ?></h5>

We shouldn't be rendering empty h5.top-tags in themes.