Fieldable entities without bundles throw an exception.

Problem/Motivation

With the Workbench Moderation module enabled you can no longer edit fieldable entities that don't have bundles.

Steps to reproduce

1. Enable the Workbench Moderation module.
2. Enabled the Paragraphs Library sub-module that comes with the Paragraphs module.
3. Go to /admin/content/paragraphs and click Add Library Item.

You'll receive the standard "The website encountered an unexpected error. Please try again later." error and if you check in the logs you'll see an error that reads Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" entity type does not exist.

Proposed resolution

The code causing the problem is in src/Plugin/Field/FieldWidget/ModerationStateWidget.php. It's attempting to load the entity type from storage, but is assuming it has a bundle.

$bundle_entity = $this->entityTypeManager->getStorage($entity->getEntityType()->getBundleEntityType())->load($entity->bundle());

Since "$entity->getEntityType()->getBundleEntityType()" is empty, it generates the above error. Proposed resolution is to simply check if the bundle entity type is blank and use the entity type id function instead for that situation. That's what the attached patch does.