BUG/MINOR: stream: init variables when the list is empty

BUG/MINOR: stream: init variables when the list is empty

We need to call vars_init() when the list is empty otherwise we
can't use variables in the response scope. This regression was
introduced by cda7f3f5 (MINOR: stream: don't prune variables if
the list is empty).

The following config reproduces the issue:

 defaults
   mode http

 frontend in
   bind *:11223
   http-request set-var(req.foo) str("foo")  if { path /bar }
   http-request set-header bar %[var(req.foo)]  if { var(req.foo) -m found }
   http-response set-var(res.bar) str("bar")
   http-response set-header foo %[var(res.bar)] if { var(res.bar) -m found }
   use_backend out

 backend out
   server s1 127.0.0.1:11224

 listen back
   bind *:11224
   http-request deny deny_status 200

 > GET /ba HTTP/1.1
 > Host: localhost:11223
 > User-Agent: curl/7.66.0
 > Accept: */*
 >
 < HTTP/1.0 200 OK
 < Cache-Control: no-cache
 < Content-Type: text/html

 > GET /bar HTTP/1.1
 > Host: localhost:11223
 > User-Agent: curl/7.66.0
 > Accept: */*
 >
 < HTTP/1.0 200 OK
 < Cache-Control: no-cache
 < Content-Type: text/html
 < foo: bar

This must be backported as far as 1.9.