fix(useCookie): preserve empty cookie values

[feritcemkahraman]

Description

An empty cookie becomes null when the component remounts, even though useCookie accepts '' when setting it. Use a nullish fallback so an empty cookie stays empty and only a missing cookie becomes null.

The tests cover reading an existing empty cookie and setting one before remounting. The docs now explain that distinction. Cookie-name changes are handled separately in #2723.

Checked with the original Jest setup: 494 tests in 76 suites pass, including all six cookie tests. Full lint passes with 97 existing warnings. The changed hook passes a strict TypeScript check; full type/build checks still fail in the existing nano-css/csstype dependency declarations.

Validation details

Both new tests fail before the fix. Hook coverage is 100% statements/functions/lines and 90.9% branches, including transpiler branches. Jest 26, React 17.0.2 and js-cookie 3.0.5 were used. The full lint run used an LF-normalized local Windows checkout.

The original yarn.lock Babel types were used to get past newer syntax unsupported by TypeScript 4.1.5; three nano-css/csstype declaration errors remain. No package or lockfile changes are included. git diff --check passes. Storybook was not built.

Type of change

• Bug fix

Checklist

• Read the Contributing Guide
• Perform a code self-review
• Comment the code, particularly in hard-to-understand areas
• Add documentation
• Add hook's story at Storybook
• Cover changes with tests
• Ensure the test suite passes (yarn test)
• Provide 100% tests coverage
• Make sure code lints (yarn lint).
• Make sure types are fine (yarn lint:types).