Unable to create a new entity with behaviour "private" (frontend)

The behavior ignores the fact that the FOFTable `$record` could be empty (i.e. a new record).

You can replicate that problem:

1. Define an entity with behavior `private`
2. Frontend login with a user with `core.create` permission for the entity
3. Try to access the entity's `add` task
4. You get an empty page, without any form