osb/app/Casts/CollectionOrNull.php

<?php

namespace App\Casts;

use Illuminate\Contracts\Database\Eloquent\CastsAttributes;
use Illuminate\Database\Eloquent\Model;

class CollectionOrNull implements CastsAttributes
{
	/**
	 * Cast the given value.
	 *
	 * @param  array<string, mixed>  $attributes
	 */
	public function get(Model $model, string $key, mixed $value, array $attributes): mixed
	{
		return collect(json_decode($value, true));
	}

	/**
	 * Prepare the given value for storage.
	 *
	 * @param  array<string, mixed>  $attributes
	 */
	public function set(Model $model, string $key, mixed $value, array $attributes): mixed
	{
		return count($value) ? json_encode($value) : NULL;
	}
}