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("HelloWorld")=="helloworld"
  assert str_to_lower("HELLO WORLD")=="hello world"
  assert str_to_lower("123ABCxyz")=="123abcxyz"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      if s == "":
          return ""
      return s.lower()