Message Error UnauthorizedHttpException

| Q | A
| ----------------- | ---
| Bug? | no
| New Feature? | no
| Framework | Laravel
| Framework version | 5.7
| Package version | 2.0.0-alpha2
| PHP version | 7.2

#### Actual Behaviour

When the user is not authenticated, the error appears:
"message": "Type error: Argument 1 passed to Symfony\Component\HttpKernel\Exception\UnauthorizedHttpException::__construct() must be of the type string, null given, called in
"C:\xampp\htdocs\doc3w\vendor\dingo\api\src\Auth\Auth.php on line 113".

#### Expected Behaviour

Reviewing the code in the throwUnauthorizedException method of auth.php (vendor\dingoapi\scr\auth\auth) it appears that if the exception is null, the field is null and void and if we put "" the error appears: "message": "Failed to authenticate because of bad credentials or an invalid authorization header.",

As mentioned in previously closed issue 1548, however in this case I am already using 2.0.0-alpha2 and my laravel version was just upgraded to 5.7, this issue wasn't present prior to that update.

Cheers,
Nicole