input: Write a function to reverse the order of words in a given string. The function should take a string as input and return the string with its words in reverse order. If the input string is empty, the function should return an empty string.
  assert reverse_words("Hello world")=="world Hello"
  assert reverse_words("This is a test")=="test a is This"
  assert reverse_words("Python programming is fun")=="fun is programming Python"
  assert reverse_words("SingleWord")=="SingleWord"
  assert reverse_words("")==""
output: def reverse_words(s):
      return ' '.join(s.split()[::-1]) if s else ''