input: Write a function to combine two given strings and return the reversed result. If either string is empty, return the non-empty string reversed. If both strings are empty, return an empty string.
  assert reverse_combined("", "world!") == "!dlrow"
  assert reverse_combined("Hello ", "") == " olleH"
  assert reverse_combined("Hello ", "world!") == "!dlrow olleH"
  assert reverse_combined("", "") == ""
output: def reverse_combined(s1, s2):
      if not s1 and not s2:
          return ""
      elif not s1:
          return s2[::-1]
      elif not s2:
          return s1[::-1]
      else:
          return (s1 + s2)[::-1]