[#420] follow-up: also handle the case when current user is nil

app/views/shared/_restricted_message.html.erb

@@ -1,4 +1,4 @@
-<% if current_user.guest %>
+<% if current_user.nil? || current_user.guest %>
 <div class="alert alert--warning" role="alert">
   <button type="button" class="close" data-dismiss="alert" aria-label="click to close" role="button">
     <span aria-hidden="true">×</span>