input: Write a function to identify the longest word in a given sentence. If the sentence is empty, return None.
  assert find_longest_word("The rain in Spain stays mainly in the plain.")=="mainly"
  assert find_longest_word("Python programming is interesting.")=="programming"
  assert find_longest_word("This is a simple test.")=="simple"
  assert find_longest_word("")==None
  assert find_longest_word("One two three four five")=="three"
output: def find_longest_word(sentence):
      if not sentence:
          return None
      words = sentence.split()
      return max(words, key=len)