fix build error (see also chuckhoupt#24)

JekyllHelp/_includes/header.html

@@ -8,7 +8,7 @@
 		{% assign topics_tree = site[page.collection] | tree_by_with_prop_lookup: "group", "order", title_page.group_order | stable_sort: "order" %}
 		<ul>
 		{% for node in topics_tree %}
-			{% if node.name %}
+			{% if node.items %}
 				{% assign sorted_topics = node.items | sort: "order" %}
 				<li class="group">
 					<a name="name"><nav>{{ node.name }}</nav></a>