Fix main menu: don't create empty menu w/o items

layouts/partials/header.html

@@ -1,10 +1,12 @@
 <header>
 	<a href="{{ .Site.BaseURL }}">{{ .Site.Title }}</a>
+	{{ with .Site.Menus.main }}
 	<nav>
 		<ul>
-			{{ range .Site.Menus.main }}
+			{{ range . }}
 			<li><a href="{{ .URL | relURL }}">{{ .Name }}</a></li>
 			{{ end }}
 		</ul>
 	</nav>
+	{{ end }}
 </header>