fix(useCookie): preserve empty cookie values

docs/useCookie.md

@@ -34,6 +34,9 @@ const Demo = () => {
 
 ## Reference
 
+The returned value is `null` when the cookie does not exist. An existing cookie with
+an empty value is returned as an empty string (`''`).
+
 ```ts
 const [value, updateCookie, deleteCookie] = useCookie(cookieName: string);
 ```

src/useCookie.ts

@@ -4,7 +4,7 @@ import Cookies from 'js-cookie';
 const useCookie = (
   cookieName: string
 ): [string | null, (newValue: string, options?: Cookies.CookieAttributes) => void, () => void] => {
-  const [value, setValue] = useState<string | null>(() => Cookies.get(cookieName) || null);
+  const [value, setValue] = useState<string | null>(() => Cookies.get(cookieName) ?? null);
 
   const updateCookie = useCallback(
     (newValue: string, options?: Cookies.CookieAttributes) => {

tests/useCookie.test.tsx

@@ -1,9 +1,36 @@
 import { renderHook, act } from '@testing-library/react-hooks';
 import Cookies from 'js-cookie';
-import { useCookie } from '../src';
+import useCookie from '../src/useCookie';
 
 const setup = (cookieName: string) => renderHook(() => useCookie(cookieName));
 
+it('should preserve an existing cookie with an empty value', () => {
+  const cookieName = 'empty-cookie';
+  Cookies.set(cookieName, '');
+
+  const { result, unmount } = setup(cookieName);
+
+  expect(result.current[0]).toBe('');
+  unmount();
+  Cookies.remove(cookieName);
+});
+
+it('should preserve an empty value after updating and remounting', () => {
+  const cookieName = 'empty-updated-cookie';
+  const { result, unmount } = setup(cookieName);
+
+  act(() => {
+    result.current[1]('');
+  });
+  expect(result.current[0]).toBe('');
+  unmount();
+
+  const remounted = setup(cookieName);
+  expect(remounted.result.current[0]).toBe('');
+  remounted.unmount();
+  Cookies.remove(cookieName);
+});
+
 it('should have initial value of null if no cookie exists', () => {
   const { result } = setup('some-cookie');