input: Write a function to list the contents of a directory. The function should take the directory path as an argument and return a list of its contents. If the directory does not exist, return an empty list.
  assert list_directory_contents('/home/user/documents')==['file1.txt', 'file2.txt', 'folder1']
  assert list_directory_contents('/home/user/non_existent_folder')==[]
  assert list_directory_contents('/')==['bin', 'boot', 'dev', 'etc', 'home', 'lib', 'media', 'mnt', 'opt', 'proc', 'root', 'run', 'sbin', 'srv', 'sys', 'tmp', 'usr', 'var']
  assert list_directory_contents('')==[]
output: import os
  
  def list_directory_contents(directory_path):
      if not os.path.exists(directory_path):
          return []
      return os.listdir(directory_path)