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):
      new_string = ""
  
      for char in string:
          new_string += char.lower()
  
      return new_string
  assert str_to_lower("HELLO") == "hello"
  assert str_to_lower("World") == "world"
  assert str_to_lower("123ABC") == "123abc"
  assert str_to_lower("") == ""
output: def str_to_lower(string):
      return string.lower()