`EntityOwnerInterface::getOwner()` should have nullable return value

Thanks for the issue, was about to create this myself after a PHPStan strict run on our codebase. Adding tag.

Just two things

Do you mind adding tests for these?

getOwner may be missing due to a deleted user, so we should document that. I created a very similar issue for getRevisionUser over at 🐛 RevisionLogInterface is typehinted as always returning entity/ids, but cannot guarantee set/existing values Fixed / MR2754, and received similar feedback.

I'm not sure if I agree with this. The bug is in implementation. Implementers should return an anonymous user if the referenced user entity in inaccessible. See the Commerce interface method getCustomer which we used over getOwner for orders: https://git.drupalcode.org/project/commerce/-/blob/8.x-2.x/modules/order...

The implementation:

  /**
   * {@inheritdoc}
   */
  public function getCustomer() {
    $customer = $this->get('uid')->entity;
    // Handle deleted customers.
    if (!$customer) {
      $customer = User::getAnonymousUser();
    }
    return $customer;
  }

I'm also not sure I agree. There are hooks in Drupal that should be implemented to properly delete user data. If that data should be persisted then the system should think about what it should do to keep the data integrity alive.

Currently the EntityOwnerInterface provides a contract to code to say "This thing is owned by something". Making getOwner() possibly return NULL would drastically change the meaning of the interface to EntityMaybeOwnerInterface.

From a business logic perspective I'd have to handle the NULL case significantly different from the non-null case.

I agree with Matt (#6) that the anonymous user would make a good fallback to indicate: "We don't know who this user is". This already happens with nodes that are assigned to the anonymous user.

For Open Social's Real-Time Chat it was important that data integrity was retained since a user may have multiple conversations with different deleted users. Making all of those NULL or the anonymous user would've made it difficult to distinguish between them. Therefore we implemented logic to ensure the data integrity as it made sense for our application to remain intact. We've outlined the work for this and to port it from our proprietary extension to the distribution in #3221021: [Meta] Handle data integrity for deleted users and prepare for non-human actions → .

This sounds good, though we're going to need to make sure implementations dont presently return NULL. This will certainly be the case with \Drupal\user\EntityOwnerTrait as ->entity computed property is nullable