The API key associated with your account is for FlightXML v2, but the fxmlURL in the example used is for FlightXML v3. This will cause the authentication to fail and the response status code to be a 401. If your example only prints a result on a status code 200 occurs then this is why there is no output.
Please try changing the fxmlUrl the value to https://flightxml.flightaware.com/json/FlightXML2/ and update the request to match the format of one of the FlightXML v2 requests found here. You may also want to add a condition to print the error if a status code 200 is not returned.