Fix TypeScript return type

Fixes #17

Author: sindresorhus

index.d.ts

@@ -30,7 +30,7 @@ console.log(await packageDirectory());
 //=> '/Users/sindresorhus/foo'
 ```
 */
-export function packageDirectory(options?: Options): Promise<string>;
+export function packageDirectory(options?: Options): Promise<string | undefined>;
 
 /**
 Synchronously find the root directory of a Node.js project or npm package.
@@ -55,4 +55,4 @@ console.log(packageDirectorySync());
 //=> '/Users/sindresorhus/foo'
 ```
 */
-export function packageDirectorySync(options?: Options): string;
+export function packageDirectorySync(options?: Options): string | undefined;

index.test-d.ts

@@ -1,5 +1,5 @@
 import {expectType} from 'tsd';
 import {packageDirectory, packageDirectorySync} from './index.js';
 
-expectType<Promise<string>>(packageDirectory({cwd: '/Users/project/pkg-dir'}));
-expectType<string>(packageDirectorySync({cwd: '/Users/project/pkg-dir'}));
+expectType<Promise<string | undefined>>(packageDirectory({cwd: '/Users/project/pkg-dir'}));
+expectType<string | undefined>(packageDirectorySync({cwd: '/Users/project/pkg-dir'}));