input: Write a function to convert a string to its lowercase version. If the string is empty, return the string as is.
  def str_to_lower(string):
      if not string:
          return string
      
      return string.lower()
  
  assert str_to_lower("HELLO WORLD")=="hello world"
  assert str_to_lower("Python Programming")=="python programming"
  assert str_to_lower("123@abc")=="123@abc"
  assert str_to_lower("")==""
  assert str_to_lower("ALL CAPS")=="all caps"
output: def str_to_lower(string):
      if not string:
          return string
      
      return string.lower()