enhancement #649

as refFilter, urlFilter is expanded checking agains url !== null.

lib/filters.js

@@ -3,7 +3,7 @@ import { removeTrailingSlash, removeWWW, getDomainName } from './url';
 
 export const urlFilter = (data, { raw }) => {
   const isValidUrl = url => {
-    return url !== '' && !url.startsWith('#');
+    return url !== '' && url !== null && !url.startsWith('#');
   };
 
   if (raw) {