input: Write a python function to delete empty directories in a given path.
  assert delete_empty_dirs('path_to_test_dir') is None
output: import os
  
  def delete_empty_dirs(path):
      for root, dirs, files in os.walk(path, topdown=False):
          for dir in dirs:
              dir_path = os.path.join(root, dir)
              if not os.listdir(dir_path):
                  os.rmdir(dir_path)