input: Write a function to combine two given strings and reverse the result. If either of the strings is empty, return the reverse of the non-empty string. The function should take two string inputs and return a single string.
  assert reverse_combined_string("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined_string("Python", "")=="nohtyP"
  assert reverse_combined_string("", "Language")=="egaugnaL"
  assert reverse_combined_string("", "")==""
  assert reverse_combined_string("Programming ", "is fun!")=="!nuf si gnimmargorP"
output: def reverse_combined_string(s1, s2):
      if s1 == "":
          return s2[::-1]
      elif s2 == "":
          return s1[::-1]
      else:
          return (s1 + s2)[::-1]