fix: return / when calling realpathSync with / (#867)

* fix #863 -- realpathSync('/') returns invalid result

-  #863

* run prettier

* style: move a test case

* refactor: inline conditional

Co-authored-by: Gareth Jones <Jones258@Gmail.com>

src/__tests__/volume/realpathSync.test.ts

@@ -1,6 +1,6 @@
 import { create } from '../util';
 
-describe('.realpath(...)', () => {
+describe('.realpathSync(...)', () => {
   it('works with symlinks, #463', () => {
     const vol = create({});
     vol.mkdirSync('/a');
@@ -11,4 +11,8 @@ describe('.realpath(...)', () => {
     const path = vol.realpathSync('/a/b/index.js');
     expect(path).toBe('/c/index.js');
   });
+  it('returns the root correctly', () => {
+    const vol = create({ './a': 'a' });
+    expect(vol.realpathSync('/')).toBe('/');
+  });
 });

src/volume.ts

@@ -1493,7 +1493,7 @@ export class Volume {
     const realLink = this.getResolvedLink(steps);
     if (!realLink) throw createError(ENOENT, 'realpath', filename);
 
-    return strToEncoding(realLink.getPath(), encoding);
+    return strToEncoding(realLink.getPath() || '/', encoding);
   }
 
   realpathSync(path: PathLike, options?: IRealpathOptions | string): TDataOut {