use an empty name as default server_name instread of hostname

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 8f1ab08..f4d0aae 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -3011,7 +3011,7 @@
     }
 
     if (conf->server_name.data == NULL) {
-        conf->server_name = cf->cycle->hostname;
+        ngx_str_set(&conf->server_name, "");
 
         sn = ngx_array_push(&conf->server_names);
         if (sn == NULL) {
@@ -3022,8 +3022,7 @@
         sn->regex = NULL;
 #endif
         sn->server = conf;
-        sn->name.len = conf->server_name.len;
-        sn->name.data = conf->server_name.data;
+        ngx_str_set(&sn->name, "");
     }
 
     return NGX_CONF_OK;