input: Write a function to identify the longest word in a given sentence. The function should split the sentence into words, iterate through each word to find the one with the maximum length, and then return that word. If there are multiple words with the same maximum length, return the first one encountered.
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("Python is a great programming language.") == "programming"
  assert find_longest_word("Short sentences also.") == "sentences"
  assert find_longest_word("") == None
  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