input: Write a function to reverse the order of words in a given string. If the string is empty, return an empty string.
  
  assert reverse_words("Hello world") == "world Hello"
  assert reverse_words("The quick brown fox") == "fox brown quick The"
  assert reverse_words("Python programming is fun") == "fun is programming Python"
  assert reverse_words("") == ""
  assert reverse_words("One") == "One"
output: def reverse_words(s):
      return ' '.join(s.split()[::-1]) if s else ''