When sending only simplepush data like:

{
  "simple-push": "version=123"
}

nothing happens.

The reason is we do only check for the message content, not simplepush:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/push/src/main/java/org/jboss/aerogear/unifiedpush/message/SenderServiceImpl.java#L100