`user.valid` should check for empty body

[jmrog]

Since stardog.js ought to know the Stardog API, it ought to know that the endpoint for the user.valid call responds with an empty body, and it therefore ought to check that this is the case. This prevents weird cases where you get a "200" response to the user.valid call that actually isn't a real success (e.g., a case where you have a web server running at localhost that responds with a 200 for arbitrary URLs, but the body isn't empty because it serves up the index file or something like that).