How to create validator that allows either null or an integer?

[piotr-cz]

How to test that array of data:

• contains given key
• key is either null or an integer?

// Case 1: Passes when key is missing
$v = new \Valitron\Validator([]);
$v->mapFieldRules('id', ['optional', 'integer']);

$v->validate(); // true
$errors = $v->errors(); // []


// Case 2: Fails for null
$v = new \Valitron\Validator(['id' => null]);
$v->mapFieldRules('id', ['required', 'integer']);

$r = $v->validate(); // false
$errors = $v->errors(); // ['id' => ['Id required', 'Id integer']]


// Case 3: Fails for null
$v = new \Valitron\Validator(['id' => null]);
$v->mapFieldRules('id', [['required', true], 'integer']);

$r = $v->validate(); // false
$errors = $v->errors(); // ['id => ['Id integer']];

I've tried creating custom validator but with no luck