input: Write a function to identify the longest word in a given sentence. If there are multiple words with the same length, return any one of them.
  assert find_longest_word("The rain in Spain stays mainly in the plain.")== "mainly"
  assert find_longest_word("Python is a fun language to learn.")== "language"
  assert find_longest_word("Testing is essential for quality.")== "essential"
  assert find_longest_word("A quick brown fox jumps over the lazy dog.")== "quick"
  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)
      return longest_word