watch: clarify that the fileName parameter can be null

Add a comment to clarify that the `fileName` parameter can be `null` if
the file name cannot be determined.

lib/internal/watch_mode/files_watcher.js

@@ -97,8 +97,11 @@ class FilesWatcher extends EventEmitter {
       return;
     }
     const watcher = watch(path, { recursive, signal: this.#signal });
-    watcher.on('change', (eventType, fileName) => this
-      .#onChange(recursive ? resolve(path, fileName ?? '') : path));
+    watcher.on('change', (eventType, fileName) => {
+      // `fileName` can be `null` if it cannot be determined. See
+      // https://github.com/nodejs/node/pull/49891#issuecomment-1744673430.
+      this.#onChange(recursive ? resolve(path, fileName ?? '') : path);
+    });
     this.#watchers.set(path, { handle: watcher, recursive });
     if (recursive) {
       this.#removeWatchedChildren(path);