Ignore calls to Ack/Nack if the doneFunc isn't set (#3139)

This allows for simple testing.

I personally prefer #2920, but I didn't win that.

Co-authored-by: Alex Hong <9397363+hongalex@users.noreply.github.com>

pubsub/message.go

@@ -119,5 +119,7 @@ func (m *Message) done(ack bool) {
 		return
 	}
 	m.calledDone = true
-	m.doneFunc(m.ackID, ack, m.receiveTime)
+	if m.doneFunc != nil {
+		m.doneFunc(m.ackID, ack, m.receiveTime)
+	}
 }