Define "empty" for containers and supporters
The "empty / non-empty" adjective is defined for many internal types (rulebooks, texts, etc) but not for world-model objects. I think it would align with authors' expectations if "empty" was defined at least for containers and supporters.
(A game could still override this adjective for specific objects if other definitions are desired.)
Be careful about efficiency. The following works without having to iterate the contents:
Definition: a container is empty rather than non-empty if the first thing held by it is nothing.