fix header existence logic, exactly treat as null or empty string

[ysugimoto]

This changes are pretty important that improves checking header key existence inside if condition.
In Fastly runtime, the not-set header, means header is not exist will be false in if condition.
However, the empty string value even header key exists, it would be true in if condition.

if (req.http.Foo) {
  log "exists"; // don't output because Foo header is not-set
}
set req.http.Foo = "";
if (req.http.Foo) {
  log "exists2"; // output because Foo header is set with empty string
}

Fastly fiddle: https://fiddle.fastly.dev/fiddle/ed2a5dbb

This bug comes from golang's zero-value relating things... it's very helpful for real world usage but not for reproducing VCL runtime X(