Add empty route to fix missing route

Closes #3419670

src/Entity/Source.php

@@ -10,10 +10,14 @@ namespace Drupal\backup_migrate\Entity;
  *   label = @Translation("Backup Source"),
  *   handlers = {
  *     "list_builder" = "Drupal\backup_migrate\Controller\SourceListBuilder",
+ *     "view_builder" = "Drupal\Core\Entity\EntityViewBuilder",
  *     "form" = {
  *       "default" = "Drupal\backup_migrate\Form\SourceForm",
  *       "delete" = "Drupal\backup_migrate\Form\EntityDeleteForm"
  *     },
+ *     "route_provider" = {
+ *       "html" = "Drupal\Core\Entity\Routing\DefaultHtmlRouteProvider",
+ *     },
  *   },
  *   admin_permission = "administer backup and migrate",
  *   entity_keys = {