input: Write a function to combine two strings and reverse the result in Python. If either of the strings is empty the function should return None.
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("Hi", "")==None
  assert combine_and_reverse("", "there!")==None
  assert combine_and_reverse("Good ", "morning!")=="!gninrom dooG "
  assert combine_and_reverse("", "")==None
output: def combine_and_reverse(str1, str2):
      if str1 == "" or str2 == "":
          return None
      combined = str1 + str2
      return combined[::-1]