Regression: Exception doing a diff on some field types

#2708601: Allow per-field diff configuration introduced a regression for fields where a plugin cannot be found when doing a diff between two revisions with certain field types. The attached patch appears to resolve it.

The specific field that failed was an og membership reference field.

The resulting exception looks like this:

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 52 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php).
Drupal\Core\Plugin\DefaultPluginManager->getDefinition(NULL) (Line: 16)
Drupal\Core\Plugin\Factory\ContainerFactory->createInstance(NULL, Array) (Line: 84)
Drupal\Component\Plugin\PluginManagerBase->createInstance(NULL, Array) (Line: 108)
Drupal\diff\DiffEntityParser->parseEntity(Object) (Line: 116)
Drupal\diff\EntityComparisonBase->compareRevisions(Object, Object) (Line: 84)
Drupal\diff\Controller\NodeRevisionController->compareNodeRevisions(Object, '179365', '180181', 'raw')
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array) (Line: 139)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 62)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 98)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 77)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 628)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)