Support entities without edit handler class

Repeatable: Always

Steps to repeat:
1. Install current dev on fresh D8.
2. Go to "/admin/structure/conditional_fields".
3. Click on "User" entity and then "User" bundle.
4. Fill some dependency and click "Add dependency".

Expected Results:
Be able to add new and edit existing conditional fields on entities, where "edit" form class is not defined.

Actual Results:
Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException: The "user" entity type did not specify a "edit" form class. in Drupal\Core\Entity\EntityTypeManager->getFormObject() (line 184 of core/lib/Drupal/Core/Entity/EntityTypeManager.php