fix(useCookie): preserve empty cookie values

[feritcemkahraman]

Description

An existing cookie with an empty value is returned as null by useCookie, because initialization uses a truthiness fallback. Updating a cookie to an empty string works until the component is remounted, when the same persisted value becomes null.

Use a nullish fallback so only a missing cookie maps to null. Add regression tests for an existing empty cookie and an empty update followed by remount, and document the empty-string/missing-cookie distinction.

This addresses value initialization independently of the cookie-name-change behavior proposed in #2723.

Validation

• Jest 26 with React 17.0.2 and js-cookie 3.0.5: both new regressions fail before the fix; all six useCookie tests pass after it.
• Targeted TypeScript 4.1.5 strict check of the changed hook passes.
• Targeted coverage: 100% statements, functions and lines; 90.9% branches (including transpiler-generated branches).
• Full repository npm test now passes: 76 suites and 494 tests, using the original Jest configuration.
• Full repository npm run lint passes with 0 errors and 97 existing warnings after normalizing the local Windows checkout to LF; no source changes were introduced.
• Full npm run lint:types and npm run build:cjs were attempted. They fail in dependency declarations: initially newer @types/babel__traverse syntax unsupported by TypeScript 4.1.5; using its yarn.lock version (7.11.0) exposes three nano-css/csstype declaration errors. No package or lockfile changes are included.
• git diff --check passes.

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).

ESLint 8 using the repository's configuration passes on both changed TypeScript files (via the isolated dependency harness).