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 then return the non-empty string in reverse order.
  assert combine_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_reverse("abc", "def")=="fedcba"
  assert combine_reverse("", "xyz")=="zyx"
  assert combine_reverse("123", "")=="321"
output: def combine_reverse(str1, str2):
      combined = str1 + str2
      if not combined:
          return ""
      return combined[::-1]