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("Python is fun")=="fun is Python"
  assert reverse_words("Testing this code")=="code this Testing"
  assert reverse_words("")==""
output: def reverse_words(s):
      return ' '.join(s.split()[::-1]) if s else ''