input: Write a function to identify the longest word in a given sentence. If the sentence is empty then return None. The function should split the sentence into words and find the word with the longest length.
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("A quick brown fox jumps over the lazy dog.") == "quick"
  assert find_longest_word("Python programming is fun.") == "programming"
  assert find_longest_word("") == None
output: def find_longest_word(sentence):
      if not sentence:
          return None
      words = sentence.split()
      return max(words, key=len)