input: Write a function to identify the longest word in a given sentence. Return the longest word itself. If there are multiple words with the same longest 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 an amazing programming language.") == "programming"
  assert find_longest_word("It is a beautiful day.") == "beautiful"
  assert find_longest_word("") == None
  assert find_longest_word("Hi") == "Hi"
  assert find_longest_word("One two three four five") == "three"
output: def find_longest_word(sentence):
      if not sentence:
          return None
      words = sentence.split()
      longest_word = max(words, key=len)
      return longest_word