Hi,
I just ran into the same bug. I would say the current behaviour is OK with SOAP responses (you should always get a response), but there are legitimate cases with REST-responses where you have no content, but rely on the HTTP status code.
I've checked out the latest code and the problem is still not solved. I suggest changing ValidHttpStatusCodesAssertion and InvalidHttpStatusCodesAssertion to allow empty responses. I cannot see any negative consequences, but this should be verified by a developer familiar with this code. See the attached patch (renamed to .txt because .patch is not allowed in this forum!!!).