Fallback to site.github.url for use in {{ base_path }} when site.url is nil

_includes/base_path

@@ -1 +1,5 @@
-{% capture base_path %}{{ site.url }}{{ site.baseurl }}{% endcapture %}
+{% if site.url %}
+  {% assign base_path = site.url | append: site.baseurl %}
+{% else %}
+  {% assign base_path = site.github.url %}
+{% endif %}