input: Write a function to identify the longest word in a given sentence. If there is a tie for the longest word, return the first one encountered. 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 fun.") == "programming"
  assert find_longest_word("A B C D E F G") == "A"
  assert find_longest_word("") == None
output: def find_longest_word(sentence):
      if not sentence:
          return None
      words = sentence.split()
      longest_word = max(words, key=len, default=None)
      return longest_word