Message Error Authenticate users

| Q | A
| ----------------- | ---
| Bug? | no|yes
| New Feature? | no|yes
| Framework | Laravel
| Framework version | 5.6
| Package version | 2.0.0-alpha1
| PHP version | 7.1.10

#### 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.",

Thx Alejandro