title: Trigger AWS Lambda once SQS fifo queue is not empty
body: <p>I got an SQS FIFO queue, I want to know if there is a way to trigger an AWS lambda once the queue is not empty.
  For example, if my queue is empty and a new message enters trigger lambda, but if the queue is already containing at least one message and a new message enters no lambda will be triggered.
  Is it possible?</p>