Non nullable parameter in ValidatorTrait

[AlexLisenkov]

Validating fields with validExistence set to false creates an exception when calling the getValidator method.

"Argument 1 passed to App\\JsonApi\\Hydrator\\JobPosition\\AbstractJobPositionHydrator::getValidator() must be of the type string, null given, called in /var/www/html/vendor/paknahad/jsonapi-bundle/src/Hydrator/ValidatorTrait.php on line 89".

Parameter is not nullable but can be null due to validExistence set to false.