Handle missing arguments

Without this, if all arguments are missing and `HTTP_X_PROXY_URL` isn't set, the server throws an error (could happen if /proxy.php is spidered for example).
This code handles that edge case.
Probably could use a 400 instead of a 404 but this at least prevents the error message.
Merge pull request #5 from audionerd/patch-1

Handle missing arguments