test: cover path empty string case

In path.toNamespacePath was a case when the path
was empty string and it wasn't covered in the tests.

I covered this case both in Windows and Unix environments.

PR-URL: #24569
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: lakatostamas

test/parallel/test-path-makelong.js

@@ -43,6 +43,7 @@ if (common.isWindows) {
                      '\\\\.\\pipe\\somepipe');
 }
 
+assert.strictEqual(path.toNamespacedPath(''), '');
 assert.strictEqual(path.toNamespacedPath(null), null);
 assert.strictEqual(path.toNamespacedPath(100), 100);
 assert.strictEqual(path.toNamespacedPath(path), path);
@@ -60,6 +61,7 @@ assert.strictEqual(path.posix.toNamespacedPath(emptyObj), emptyObj);
 if (common.isWindows) {
   // These tests cause resolve() to insert the cwd, so we cannot test them from
   // non-Windows platforms (easily)
+  assert.strictEqual(path.toNamespacedPath(''), '');
   assert.strictEqual(path.win32.toNamespacedPath('foo\\bar').toLowerCase(),
                      `\\\\?\\${process.cwd().toLowerCase()}\\foo\\bar`);
   assert.strictEqual(path.win32.toNamespacedPath('foo/bar').toLowerCase(),