Return 'undefined' if HTTP header value is "" (treated the same as missing)

Empty headers are treated as missing. However, some upstream servers
(such as Openresty, with certain module) might strip empty headers while
others (such as Nginx) do not. Rather than depending on upstream servers
to strip empty headers, this patch makes any the empty headers return
the same value as missing headers (undefined). This allows OSC to past
the latest Pedant tests.