watch: fix null fileName on windows systems

PR-URL: #49891
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/internal/watch_mode/files_watcher.js

@@ -98,7 +98,7 @@ class FilesWatcher extends EventEmitter {
     }
     const watcher = watch(path, { recursive, signal: this.#signal });
     watcher.on('change', (eventType, fileName) => this
-      .#onChange(recursive ? resolve(path, fileName) : path));
+      .#onChange(recursive ? resolve(path, fileName ?? '') : path));
     this.#watchers.set(path, { handle: watcher, recursive });
     if (recursive) {
       this.#removeWatchedChildren(path);