You'll get that error for Android if you select a silent push message type and go directly to 'Send Now' without adding custom keys or the collapse key. We are looking at modifying the workflow for creating silent push notifications to make it more clear that the Message Composer expects some non-alerting elements to be added to the payload.
In the meantime, just be sure to add either a custom key or a collapse key in the Content tab of the Message Composer and you should be able to send out the silent notification.
I hope this helps. Please let me know if you have any other questions or concerns.