Return false if success variable not defined

[cmgmyr]

On occasion, within our Laravel test suite, we'll get random errors with Ray when a request cannot be completed. All that shows is a failure because the $success variable is not defined.

ErrorException : Undefined variable $success
 /project/vendor/spatie/ray/src/Client.php:60
 /project/vendor/spatie/ray/src/Client.php:39
 /project/vendor/spatie/ray/src/Client.php:66
...

After implementing this change locally, the underlying failure was able to bubble up as expected.