input: Write a function to convert a given string to its lowercase version. If the input string is empty, return an empty string.
  assert str_to_lower("Hello World!")=="hello world!"
  assert str_to_lower("PYTHON")=="python"
  assert str_to_lower("TEST123")=="test123"
  assert str_to_lower("UPPER")=="upper"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower()