Migration fails to create instance when non-bundleable entity type exists.

Problem/Motivation

The following error occurs on `drush ms` when there is a non-bundleable entity type (in this case Linkback):

InvalidArgumentException: A bundle was provided but the entity type (linkback) is not bundleable. in                                                                                     [error]
/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php:112
Stack trace:
#0 /core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php(133): Drupal\migrate_drupal\Plugin\migrate\source\ContentEntity->__construct(Array,
'content_entity:...', Array, Object(Drupal\migrate\Plugin\Migration), Object(Drupal\Core\Entity\EntityTypeManager), Object(Drupal\Core\Entity\EntityFieldManager),
Object(Drupal\Core\Entity\EntityTypeBundleInfo))
#1 /core/modules/migrate/src/Plugin/MigratePluginManager.php(57):
Drupal\migrate_drupal\Plugin\migrate\source\ContentEntity::create(Object(Drupal\Core\DependencyInjection\Container), Array, 'content_entity:...', Array,
Object(Drupal\migrate\Plugin\Migration))
...

This is caused by the migration d6_entity_reference_translation calling EntityReferenceTranslationDeriver::getDerivativeDefinitions() and setting 'bundle' in the migration source.

Proposed resolution

Check if the entity type has bundles.