fix(open_graph): url might be null

Author: curbengh

lib/plugins/helper/open_graph.js

@@ -78,11 +78,12 @@ function openGraphHelper(options = {}) {
   result += og('og:type', type);
   result += og('og:title', title);
 
-  if (config.pretty_urls.trailing_index === false) {
-    url = url.replace(/index\.html$/, '');
+  if (url) {
+    if (config.pretty_urls.trailing_index === false) {
+      url = url.replace(/index\.html$/, '');
+    }
+    url = encodeURL(url);
   }
-
-  if (url) url = encodeURL(url);
   result += og('og:url', url);
 
   result += og('og:site_name', siteName);