input: Write a function to combine two strings and reverse the result in Python. If either of the input strings is empty, return an empty string.
  assert reverse_combined("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined("Hi", "")==""
  assert reverse_combined("", "There")==""
  assert reverse_combined("123", "456")=="654321"
  assert reverse_combined("", "")==""
output: def reverse_combined(s1, s2):
      if s1 == "" or s2 == "":
          return ""
      return (s1 + s2)[::-1]