fix: check for url presence before trying to append to it (#464)

lib/graphiti/util/link.rb

@@ -48,6 +48,7 @@ def raw_url
 
       def on_demand_links(url)
         return url unless Graphiti.config.links_on_demand
+        return unless url
 
         url << if url.include?("?")
           "&links=true"