fix(res.append): preserve empty header values

[lin-hongkuan]

This makes res.append() preserve an existing empty-string header value instead of treating it as absent.

The regression test appends to an empty Warning header and verifies the empty value remains part of the emitted header list.

Validation:

• npx mocha --require test/support/env --check-leaks test/res.append.js
• npm run lint
• git diff --check

AI-assisted: Codex helped prepare this focused change; I reviewed the diff and ran the validation above before submitting.