Add default backend

Author: valentinab25

varnish-plone6/varnish.vcl

@@ -1,7 +1,11 @@
+vcl 4.1;
+
 import std;
 import directors;
 import dynamic;
 
+backend default none;
+
 sub vcl_init {
 
   new cluster = dynamic.director(port = "<VARNISH_BACKEND_PORT>", ttl = <VARNISH_DNS_TTL>);