input: Write a function to identify the longest word in a given sentence. If there is a tie, return the first word that has the longest length.
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("The quick brown fox jumps over the lazy dog.") == "quick"
  assert find_longest_word("") == None
  assert find_longest_word("Python is a high-level programming language.") == "programming"
  assert find_longest_word("A") == "A"
output: def find_longest_word(sentence):
      if not sentence:
          return None
      words = sentence.split()
      longest_word = max(words, key=len)
      return longest_word