Validation rule for Laravel to check existence in database or pass validation if value is 0 or null

validator.php

/**
 * Validate exists_or_null
 * This rule validates exactly like the default 'exists' rule, except it passes
 * when the value is equal to 0 or null. The function should be added to the
 * boot method of the AppServiceProvider. When there's another value than
 * 0 or null, the value will be validated against the default exists
 * rule.
 */
Validator::extend(
    'exists_or_null',
    function ($attribute, $value, $parameters)
    {
        if($value == 0 || is_null($value)) {
            return true;
        } else {
            $validator = Validator::make([$attribute => $value], [
                $attribute => 'exists:' . implode(",", $parameters)
            ]);
            return !$validator->fails();
        }
    }
);

/**
 * You can validate select lists like these
 */
$category_list = CompanyCategory::withParent(0)->lists('name', 'id');
$category_list->prepend('Please select a parent category (optional)'); // this results in <option value="0">Please select a parent category (optional)</option>

/**
 * Now if we would validate the field using 'exists_or_null' instead of 'exists'
 * it would pass validation because 0 is a valid option aswel. If another
 * value was selected, that value will be validated against 'exists'
 * using the exact same parameters.
 */