Supporting empty Location URIs.

An empty string in Location was being handled specially by not sending a
Location header.  This may occur after variable resolution, so we need to
consider this scenario.

The obsolete RFC 2616 defined the Location header as consisting of an absolute
URI <https://www.rfc-editor.org/rfc/rfc2616#section-14.30>, which cannot be an
empty string.  However, the current RFC 7231 allows the Location to be a
relative URI <https://www.rfc-editor.org/rfc/rfc7231#section-7.1.2>, and a
relative URI may be an empty string <https://stackoverflow.com/a/43338457>.

Due to these considerations, this patch allows sending an empty Location header
without handling this case specially.  This behavior will probably be more
straightforward to users, too.  It also simplifies the code, which is now more
readable, fast, and conformant to the current RFC.  We're skipping an
allocation at request time in a common case such as "action": {"return": 404}

src/nxt_http.h

@@ -206,7 +206,7 @@ typedef struct nxt_http_route_addr_rule_s  nxt_http_route_addr_rule_t;
 typedef struct {
     nxt_conf_value_t                *pass;
     nxt_conf_value_t                *ret;
-    nxt_str_t                       location;
+    nxt_conf_value_t                *location;
     nxt_conf_value_t                *proxy;
     nxt_conf_value_t                *share;
     nxt_str_t                       chroot;