Field UI integration for entity types is brittle

A fieldable entity type without a bundle breaks Field UI integration for all bundles of entity types that alphabetically follow it.

Lazy bit of coding on my part.

Found by agentrickard because domain has no bundle.
It doesn't occur in core because User is the only entity type that doesn't have a bundle, and there are no fieldable entity types that come after U.

The $defaults array is never reset, so it is wrong populated into the routes that follow, breaking their placeholder expansion.

I'll write tests tonight.