[8.x] Guard against invalid guard in make:policy

[t73biz]

Problem:

When using artisan to make a policy, and an invalid guard is provided to the command, as is the case in this example,
php artisan make:policy --model=Project --guard=foo ProjectPolicy the generated code for the policy is invalid.

The $dummyUser used in the replaceModel method is set to null, which then fails to generate correctly.

Sample code generated with bad guard.

<?php

namespace App\Policies;

use App\Models\Project;
use App\Models\User;
use Illuminate\Auth\Access\HandlesAuthorization;

/**
 * Class ProjectPolicy
 * @package App\Policies
 */
class ProjectPolicy
{
    use HandlesAuthorization;

    /**
     * Determine whether the user can view any Projects.
     *
     * @param  User  $
     * @return mixed
     */
    public function viewAny(User $)
    {
        //
    }

    /**
     * Determine whether the user can view the Project.
     *
     * @param  User  $
     * @param  Project  $project
     * @return mixed
     */
    public function view(User $, Project $project)
    {
        //
    }

    /**
     * Determine whether the user can create Projects.
     *
     * @param  User  $
     * @return mixed
     */
    public function create(User $)
    {
        //
    }

    /**
     * Determine whether the user can update the Project.
     *
     * @param  User  $
     * @param  Project  $project
     * @return mixed
     */
    public function update(User $, Project $project)
    {
        //
    }

    /**
     * Determine whether the user can delete the Project.
     *
     * @param  User  $
     * @param  Project  $project
     * @return mixed
     */
    public function delete(User $, Project $project)
    {
        //
    }

    /**
     * Determine whether the user can restore the Project.
     *
     * @param  User  $
     * @param  Project  $project
     * @return mixed
     */
    public function restore(User $, Project $project)
    {
        //
    }

    /**
     * Determine whether the user can permanently delete the Project.
     *
     * @param  User  $
     * @param  Project  $project
     * @return mixed
     */
    public function forceDelete(User $, Project $project)
    {
        //
    }
}

Solution:

The solution proposed here is to check for the existence of the guard, and throw an exception if it is null.