chore: improve clsx maintenance path

[quyentonndbs]

Summary:

• Add an edge-case unit test asserting that clsx() handles nested arrays containing a mix of falsy values (false, null, undefined, 0, '') interleaved with truthy strings, ensuring falsy filtering remains correct across deep nesting — covers a path not explicitly exercised in the existing suite.
• Keep the change narrow so it is straightforward to review.

Notes:

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