chore: improve clsx maintenance path

[quyentonndbs]

Summary:

• Add edge-case unit tests covering inputs that the current suite under-exercises: deeply nested arrays mixed with falsy values (null/undefined/false/0/''), Symbol keys on object inputs (should be ignored), and class strings containing only whitespace. Each is a small, isolated assert-style test addition with no behavior change required.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.