global: use SPA_ID_INVALID to send serial id

We use the done event to send the object serial to the client. Use an
invalid object id for this and filter it out on the receiver or else
the client might get an unexpected done event.

Fixes #2253

src/modules/module-protocol-native/protocol-native.c

@@ -347,6 +347,9 @@ static int core_event_demarshal_done(void *object, const struct pw_protocol_nati
 				SPA_POD_Int(&seq)) < 0)
 		return -EINVAL;
 
+	if (id == SPA_ID_INVALID)
+		return 0;
+
 	return pw_proxy_notify(proxy, struct pw_core_events, done, 0, id, seq);
 }