input: Write a function to identify the longest word in a sentence. If there are multiple words with the same maximum length, return the first one encountered. The function should handle punctuation and capitalization.
  assert find_longest_word("The rain in Spain stays mainly in the plain.")=='mainly'
  assert find_longest_word("Python is a versatile programming language.")=='programming'
  assert find_longest_word("She sells seashells by the seashore.")=='seashells'
  assert find_longest_word("Hello, world!")=='Hello,'
  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