input: Write a function to combine 2 strings and reverse the result in Python. The function should return the reversed string. If any of the input strings is None, the function should return None.
  
  def combine_and_reverse(words, words2):
      if words is None or words2 is None:
          return None
      combined = words + words2
      return combined[::-1]
  
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("Python ", "is fun!")=="!nuf si nohtyP"
  assert combine_and_reverse(None, "test")==None
  assert combine_and_reverse("test", None)==None
  assert combine_and_reverse("", "empty")=="ytpe"
  assert combine_and_reverse("full", "")=="lluf"
output: def combine_and_reverse(words, words2):
      if words is None or words2 is None:
          return None
      combined = words + words2
      return combined[::-1]