Unity Editor menu command to select all Empty folders in the project, easily clean them up or find abandoned or forgotten folders. From FPSSample

FindEmptyFolders.cs

using System.Collections.Generic;
using UnityEditor;

public static class FindEmptyFolders
{
    [MenuItem("Assets/Find Empty Folders")]
    static void OnFindEmptyFolders()
    {
        var empties = new List<UnityEngine.Object>();
        foreach (var d in System.IO.Directory.GetDirectories("Assets", "*", System.IO.SearchOption.AllDirectories))
        {
            if (System.IO.Directory.GetFiles(d).Length == 0)
            {
                empties.Add(AssetDatabase.LoadAssetAtPath<UnityEngine.Object>(d));
            }
        }
        Selection.objects = empties.ToArray();
    }
}