Rule validation for JSON throws TypeError on PHP 8.0

[Joorren]

• Laravel Version: 8.21.0 (latest)
• PHP Version: 8.0.0
• Database Driver & Version: MySQL 5.7

Description:

When using the 'JSON' rule validation in PHP 8, you will get a TypeError when submitting an empty field:

method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given
in vendor/laravel/framework/src/Illuminate/Validation/Concerns/ValidatesAttributes.php:1149

According to php.watch:
Union types are used when they are deemed necessary. For example, method_exists() function accepts either a class name (string) or an object (object). This is enforced as a Union Type of string|object. [...] Prior to PHP 8, it returned false if the provided parameter is not a string or an object, but did not raise any warnings.

Steps To Reproduce:

1. Create a form with an input field that contains no value (hidden in my case).
2. Add a json rule for this field (json|required in my case).
3. Submit the form.
   The website will return method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given