Why: the backend store List* functions encoded empty results as JSON null,
which crashed the frontend nav (.map on null) on any newly created project.
Evidence: backend go test all packages pass; store regression
TestEmptyListsSerializeAsEmptyArrays asserts [] for empty lists.
Impact: parent pins backend at the serialization fix plus the core-pin
build-blocker advance.
Refs: .task/fix-empty-list-json.md