input: Write a function to combine two given strings and return the reversed result. The function should take two string parameters and 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("Hello ", "")==" olleH"
  assert combine_and_reverse("", "world!")=="!dlrow"
  assert combine_and_reverse("", "")==""
output: def combine_and_reverse(str1, str2):
      if str1 == "":
          return str2[::-1]
      elif str2 == "":
          return str1[::-1]
      else:
          return (str1 + str2)[::-1]