Add default values for config variables (#621)

_includes/nav.html

@@ -9,7 +9,7 @@
       </button>
       {%- if site.title-img -%}
         <a class="navbar-brand navbar-brand-logo" href="{{ '' | absolute_url }}"><img src="{{ site.title-img }}"/></a>
-      {%- else -%}
+      {%- elsif site.title -%}
         <a class="navbar-brand" href="{{ '' | absolute_url }}">{{ site.title }}</a>
       {%- endif -%}
     </div>