Issue #3453786 by soham sengupta, drupatz, joachim, smustgrave,...

    return [

      // For the 'target_bundles' setting, a NULL value is equivalent to "allow

      // entities from any bundle to be referenced" and an empty array value is

      // equivalent to "no entities from any bundle can be referenced".

      // equivalent to "no entities from any bundle can be referenced". The

      // reason for NULL and an empty array having a different meaning is to

      // correctly handle config updates.

      // For example, in the scenario where a single target bundle is allowed,

      // and that bundle is then deleted, the automatic removal of that bundle

      // from the entity reference field's settings leaves an empty array.

      // This empty array must therefore indicate that no bundles are allowed,

      // as otherwise the field would suddenly allow all bundles.

      'target_bundles' => NULL,

      'sort' => [

        'field' => '_none',