input: Write a function to combine two given strings and return the result in reverse order. If any of the strings is empty, the function should return the non-empty string in reverse order. If both strings are empty, the function should return an empty string.
  
  assert reverse_combined_strings("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined_strings("Hello ", "")==" olleH"
  assert reverse_combined_strings("", "world!")=="!dlrow"
  assert reverse_combined_strings("", "")==""
output: def reverse_combined_strings(str1, str2):
      if not str1 and not str2:
          return ""
      elif not str1:
          return str2[::-1]
      elif not str2:
          return str1[::-1]
      else:
          return (str1 + str2)[::-1]