Return null instead of false in Auth0UserProvider.

[afreakk]

Changes

Return null instead of false from validateCredentials, retrieveByToken and retrieveByCredentials, in Auth0UserProvider.

Why

Because of this function in Illuminate\Auth\GuardHelpers:

    /**
     * Determine if the current user is authenticated.
     *
     * @return bool
     */
    public function check()
    {
        return ! is_null($this->user());
    }

This caused issues for us when we migrated to auth0, as some of our users utilized remember_token in previous login system.
Which caused Auth0UserProvider retrieveByToken to be called, and returned false user, which laravel counts as a "logged in user".
Which then caused exception when code asked for $user->stuff on a boolean.

In our case, only retrieveByToken was called, but it makes sense to me that retrieveByCredentials and validateCredentials also should return null instead of false.

[joshcanhelp]

Thank you @afreakk! We'll review this as soon as we can. Appreciate the contribution!

[joshcanhelp]

LGTM! Thank you for this 🙇