Fixed nil pointer dereference error when the Message == nil

[tmatsuo]

No description provided.

[tmatsuo]

I encountered a panic because of this. Can you take a look?

[rakyll]

Why is there a resp.PubsubEvent with a nil message btw?

LGTM

[tmatsuo]

Yeah, I don't know ;)

[tmatsuo]

I figured it out why.

It happens when you publish an empty message which doesn't have data nor any labels.
Maybe we should do either of

1. to forbid publishing such messages
   or
2. return empty message when receiving such messages

[tmatsuo]

No I was wrong. It also happens when you publish a message with nil data, even you provide a label.
I think the easiest fix is convert nil to []byte{} when publishing. What do you think?

[rakyll]

I wouldn't touch the existing code, a nil message may represent a distinct case than a []byte{} message. It was unexpected for me to receive a nil message, because I didn't know you were able to publish them.

[tmatsuo]

Yeah, the problem is that with this library, you actually can not receive the nil message because pull returns an error.

[tmatsuo]

Maybe we can return a message with nil Data instead of returning an error?