EntityReferenceFieldNormalizer can improperly normalize items when a reference to an internal entity is involved.

Discovered in #58 of #3015438: Wrap entity objects in a ResourceObject which carries a ResourceType.

Postponed on #3024896: Remove Relationship and RelationshipItem classes because that issue will delete the offending code, making this outdated.

EntityReferenceFieldNormalizer has a pair of arrays which must be kept in sync when serializing entity reference items:

$entity_list[] = NULL;
$entity_list_metadata[] = [

However, in it, it also does this (pseudo-code):

      $entity_list_metadata[] = $metadata;
      // ...
      if ($entity->isInternal()) {
        continue;
      }
      $entity_list[] = $entity;

This means that if the entity is internal, the field's metadata is added to one array but the entity object is not added to the other. This will create a mismatch that will cause the JSON:API output for an entity reference field to be incorrect when an entity reference item that references an internal entity precedes one that does not.