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