Add fallback site title since og:title is required.

Author: exoego

_includes/headertop.html

@@ -15,7 +15,7 @@
     <meta property="og:site_name" content="{{ site.title }}"/>
     <meta property="og:type" content="{% if page.url == '/' %}website{% else %}article{% endif %}"/>
     <meta property="og:image" content="{{ site.url }}/resources/img/scala-spiral-3d-2-toned-down.png"/>
-    {% if page.title %}<meta property="og:title" content="{{ page.title }}"/>{% endif %}
+    <meta property="og:title" content="{% if page.title %}{{ page.title }}{% else %}{{site.title}}{% endif %}"/>
     {% if page.description %}<meta property="og:description" content="{{ page.description }}"{% endif %}
 
     <meta name="twitter:card" content="summary"/>