input: Write a function to optimize a given Python script that reverses a string. If the given string is empty then return an empty string.
  assert reverse_string("hello")=="olleh"
  assert reverse_string("world")=="dlrow"
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("12345")=="54321"
  assert reverse_string("")=""
output: def reverse_string(s):
      return s[::-1] if s else ""