Fix undefined method `sort_by' error from vhost_location_empty.erb

In the template, we should check if the variable is defined before use it

templates/vhost/vhost_location_empty.erb

@@ -1,5 +1,5 @@
   location <%= @location %> {
-<% @location_custom_cfg.sort_by {|k,v| k}.each do |key,value| -%>
+<% if @location_custom_cfg -%><% @location_custom_cfg.sort_by {|k,v| k}.each do |key,value| -%>
     <%= key %> <%= value %>;
-<% end -%>
+<% end -%><% end -%>
   }