Fix header logo link on 404 page (and other pages with no html lang)

Author: jake-low

_includes/header.html

@@ -1,5 +1,7 @@
 <header>
-  <a href="/{{ page.lang }}/"><img src="/assets/logo.svg" width="225" alt="MapRoulette"/></a>
+  <a href="{% if page.lang %}/{{ page.lang }}/{% else %}/{% endif %}">
+    <img src="/assets/logo.svg" width="225" alt="MapRoulette"/>
+  </a>
   <div class="right">
     <div id="docsearch"></div>
     {% if page.variants.size > 1 %}