castAttribute does not handle nullable immutable date times

[TimothyBJacobs]

• Laravel-mongodb Version: 4.1.1
• PHP Version: 8.1
• Database Driver & Version: 6.0.13

Description:

The castAttribute method is overloaded in the base Model class. Unlike in the Core Laravel hasAttributes trait, it is missing handling for null values. See this code snippet from Laravel:

if (is_null($value) && in_array($castType, static::$primitiveCastTypes)) {
    return $value;
}

Steps to reproduce

class MyModel extends \MongoDB\Laravel\Eloquent\Model {
    protected $casts = [
        'my_date' => 'immutable_datetime',
    ];
}

$model = new MyModel();
$model->my_date; // Should be null, instead is current time.

Expected behaviour

null should be returned.

Actual behaviour

The current time is returned.