input: Write a function to convert a given string to its lowercase version. If the given string is empty, return an empty string.
  assert str_to_lower("Hello World")=="hello world"
  assert str_to_lower("PYTHON IS FUN")=="python is fun"
  assert str_to_lower("12345")=="12345"
  assert str_to_lower("")==""
  assert str_to_lower("AaBbCc")=="aabbcc"
output: def str_to_lower(s):
      return s.lower() if s else ""