Should check packet.id for non-null, not 0

Like the server does!!! (line 288 in socket.io/lib/socket.js)

Question based on engine.io-protocol documentation:

Is there an underlying problem with the packet.id being 0? Should an ack message be sent to the client first?

Inspiration for checking for non-null instead of 0 was seeing that the server code did it -- but maybe there is a deeper problem we should think about?