[core] accept "HTTP/2.0", "HTTP/3.0" from backends (#3031)

accept "HTTP/2.0" and "HTTP/3.0" NPH from naive non-proxy backends

(thx flynn)

x-ref:
  "uwsgi fails with HTTP/2"
  https://redmine.lighttpd.net/issues/3031

src/mod_scgi.c

@@ -183,7 +183,7 @@ static int scgi_env_add_uwsgi(void *venv, const char *key, size_t key_len, const
 	size_t len;
 	uint16_t uwlen;
 
-	if (!key || !val) return -1;
+	if (!key || (!val && val_len)) return -1;
 	if (key_len > USHRT_MAX || val_len > USHRT_MAX) return -1;
 
 	len = 2 + key_len + 2 + val_len;