Handle the change with netmapper and varnish 3.0.4 & later

Varnish has changed the way they handle the result of the vmod
function calls: after 3.0.4, a vmod function that returns null is
automatically changed to return an empty string.

This has also been verified by by Brandon, who found this related bug
https://www.varnish-cache.org/trac/ticket/1218

Now, with the future migration to 3.0.5 and beyond, netmapper will
always sets the resulting header as "", not NULL, and
if(response) will always be true.

We could obviously change all the other code to check for =="",
but I think keeping implicit bool-ness of headers is cleaner.

Change-Id: I7cd4518df540467b84a6ea4e5c246623a0768024