changing test to see if no messages left in queue

When there are no messages left on the queue, looks like ```aws --output=json sqs receive-message --queue-url $QUEUE_URL``` returns an empty string, and ```$NUM``` ends up also being the empty string, giving a warning when you try to compare it to 0 and 1 to check how many messages there are.

I'm not sure what the expected behavior of jq is when you pass an empty string. Maybe it is better to test before that point whether ```$JSON``` is empty?