input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged. If the given sentence is empty then return the empty string.
  
  assert reverse_words("This is a sentence.") == "sentence. a is This"
  assert reverse_words("Python is fun") == "fun is Python"
  assert reverse_words("Hello World") == "World Hello"
  assert reverse_words("") == ""
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)