input: Write a function to convert a given string to its lowercase version. If the given string is empty then return an empty string.
  assert str_to_lower("Hello World") == "hello world"
  assert str_to_lower("Python3 IS c00L") == "python3 is c00l"
  assert str_to_lower("99 BOTTLES OF BEER") == "99 bottles of beer"
  assert str_to_lower("") == ""
output: def str_to_lower(s):
      return s.lower() if s else ""