[8.x] Fix error when serializing resources with appended attributes

[duncanmcclean]

This pull request fixes an error when attempting to serialize Runway's Resource object, which ultimately attempts to serialize a fresh instance of the Eloquent model.

However, since it serialises the fresh instance of the Eloquent model, no attributes are set, so model accessors using the $attributes parameter error out:

// app/Models/User.php

protected $appends = ['thingy'];

protected function thingy(): Attribute
{
    return Attribute::make(
        get: fn (mixed $value, array $attributes) => $attributes['name']
    );
}

use App\Models\User;

(new User)->toArray();

Undefined array key "name"

To prevent an error when serializing appended attributes on a fresh model, we're now simply returning the model's class name, rather than calling its toArray() method.

Fixes #685.

[github-actions]

Released as part of v8.4.0.