The response will remain blank until it gets one from the server.
Some servers are extremely sensitive and will essentially ignore invalid requests and give you a nullpointererror on the client side. If you are the tester for this service, I would recommend filing a defect for it as the error messages can assist developers, and testers, tremendously.
Check the bottom left hand portion of the editor, see what it says. It will likely be something along the lines of response time: XXXms or it will be some sort of exception. In order to fix this, get a valid request with valid credentials from your development team and then you can progress from there.
You may be having issues with other things like auth type etc etc. There is several things that can cause the request to fail. If you supply more information it will be a lot easier to assist you.