Fix payloads for chat:typing messages with spachannel

`chat:typing` messages are broken when using SPAs with "spachannel" set. The issue is that the `SPAChannelMessage` payload doesn't allow for an optional `message` parameter.

I spoke with NiKo about this, and we decided to let the `message` parameter be `undefined` but that isn't quite going to work. So what I have done is to make `message` as an empty object when we don't need it.

That way, we can support `chat:typing` messages for now. When we redo the events system, we can reconsider breaking these into separate messages.