Summary: - Add edge-case unit tests covering inputs the current suite under-exercises: deeply nested arrays mixed with falsy values (null/undefined/false/0/''), Symbol keys on objects being ignored, and numeric class names like 0 vs non-zero numbers being handled per documented behavior. Pure additive test coverage in test/index.js with no behavior change. - Keep the change narrow so it is straightforward to review.

Notes: - I kept this scoped to the relevant implementation and tests.