navbar: reject entries with nil titles rather than empty

The blank entry in the navbar was supposed to be fixed by
c7afa11, but it didn't
work. This is presumably because the title there is nil, not
the empty string (but I can't reproduce locally, so I'm just
guessing).

Let's switch out the empty-string test for a "truthy" one
(technically we could do both, but if the only way to get an
empty string is by explicitly saying "title: " in the front
matter, it doesn't seem likely to happen).

Author: peff

_includes/navbar.html

@@ -3,7 +3,7 @@
     <li><a href="/">Home</a>
     <li><a href="/about">About</a>
     {% for node in site.pages reversed %}
-      {% if node.navbar != false and node.title != "" %}
+      {% if node.navbar != false and node.title %}
       <li><a href="{{node.url}}">{{node.title}}</a>
       {% endif %}
     {% endfor %}