input: Write a function to combine two strings and reverse the result in Python. The function should return the reversed combined string. If either of the input strings is empty, the function should return the non-empty string reversed.
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("", "world!")=="!dlrow"
  assert combine_and_reverse("Hello ", "")==" olleH"
  assert combine_and_reverse("", "")==""
output: def combine_and_reverse(str1, str2):
      combined = str1 + str2
      if combined:
          return combined[::-1]
      return ""