Merge pull request #6151 from zpao/docs-external-sidebar-followup

Docs external sidebar followup
(cherry picked from commit 4da7e7ef9c)

docs/_plugins/sidebar_item.rb

@@ -11,7 +11,7 @@ module Jekyll
       if item["href"]
         classes.push("external")
       end
-      className = classes.size ? "class=\"#{classes.join(' ')}\"" : "";
+      className = classes.size > 0  ? " class=\"#{classes.join(' ')}\"" : ""
 
       return "<a href=\"#{href}\"#{className}>#{item["title"]}</a>"
     end