input: Write a function to determine if a given sentence is 'Affirmative' or 'Negative' by identifying negation words in the sentence. If the sentence contains any negation words, it should be classified as 'Negative'; otherwise, 'Affirmative'. If the input sentence is empty, the function should return None.
  import re 
  import nltk
  nltk.download('stopwords')
  from nltk.corpus import stopwords 
  
  # Function to find the negation words
  def findNegation(sentence):
      # save all the negation words
      negationwords = {'not', 'never', 'no', 'none', 'non', 'neither',
                       'nobody', 'nowhere', 'cannot', 'couldnt', 'shouldnt', 
                       'wouldnt', 'dont', 'didnt', 'hasnt', 'havent', 'isnt', 'arent'} 
      words = nltk.word_tokenize(sentence)
      neg = []
      for word in words:
          if word in negationwords:
              neg.append(word)
      return neg
  
  # Function to check if the sentence is affirmative or negative
  def isAffirmativeorNegative(sentence):
      if not sentence:
          return None
      negationwords = findNegation(sentence)
      if len(negationwords)==0: 
          return "Affirmative" 
      else: 
          return "Negative"    
  
  assert isAffirmativeorNegative("He always comes late but I dont mind.") == "Negative"
  assert isAffirmativeorNegative("She is happy with the results.") == "Affirmative"
  assert isAffirmativeorNegative("No one could believe it.") == "Negative"
  assert isAffirmativeorNegative("") == None
output: def isAffirmativeorNegative(sentence):
      if not sentence:
          return None
      negationwords = findNegation(sentence)
      if len(negationwords)==0: 
          return "Affirmative" 
      else: 
          return "Negative"