HACK: make Node-ID non-mandatory to accomodate some non-standard server

eUPF omits the Node-ID when the Session Establishment Response reports a
failure.

This patch is bad because all current callers need to adjust, and make
sure to set the newly added node_id_present flag to true.

See also https://github.com/edgecomllc/eupf/issues/539

src/libosmo-pfcp/gen__pfcp_ies_auto.c

@@ -293,7 +293,7 @@ static const struct osmo_gtlv_gen_ie_o ies_in_msg_session_est_req[] = {
 };
 
 static const struct osmo_gtlv_gen_ie_o ies_in_msg_session_est_resp[] = {
-	M(ALL_FROM_NAME, "node_id"),
+	O(ALL_FROM_NAME, "node_id"),
 	Ms(cause),
 	Os(offending_ie),
 	O(f_seid, "up_f_seid"),

tests/libosmo-pfcp/pfcp_test.c

@@ -273,6 +273,7 @@ struct osmo_pfcp_msg tests[] = {
 			.seid = 0x0123456789abcdef,
 		},
 		.ies.session_est_resp = {
+			.node_id_present = true,
 			.node_id = {
 				.type = OSMO_PFCP_NODE_ID_T_IPV4,
 				.ip.u.sin = {