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