input: Write a function to convert a given string to its lowercase version. If the input 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 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()