Only set security.limit_extensions when not empty

Before we got data types, $security_limit_extensions was an optional
array of string defaulting to undef.  It is now an array of string
defaulting to an empty array.

We do not want to emit security.limit_extensions when this array is
empty.

templates/fpm/pool.conf.erb

@@ -378,6 +378,6 @@ php_admin_flag[<%= key %>] = <%= flag %>
 <%= line.gsub "{", "%{" %>
 <% end -%>
 
-<% if @security_limit_extensions -%>
+<% unless @security_limit_extensions.empty? -%>
 security.limit_extensions = <%= @security_limit_extensions.join(" ") %>
 <% end -%>