input: Write a function to convert a given string to its lowercase version. If the string is empty, return an empty string. 
  
  assert str_to_lower("HELLO WORLD")=="hello world"
  assert str_to_lower("aBcDeF")=="abcdef"
  assert str_to_lower("12345!@#$%")=="12345!@#$%"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower() if s else ""