Config entity types MUST allow NULL value for any config entity PHP class property corresponding to config entity property

Problem/Motivation

Discovered in #3444417: "Developer-created components": mark which SDCs should be exposed in XB.

#3364109: Configuration schema & required values: add test coverage for `nullable: true` validation support added ConfigEntityValidationTestBase::testRequiredPropertyValuesMissing().

One of the things it does:

…
    foreach ($config_entity_properties as $property) {
…
      $this->entity = clone $original_entity;
      $this->entity->set($property, NULL);

👆 This fails if your config entity's PHP class has a class prop (corresponding to a config entity prop) that is strictly typed:

  protected string $label;

rather than the historically typical:

  /**
    * @var string
    */
  protected $label;

This should be supported, but the base test makes that impossible.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet