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("Hello, World!")=="hello, world!"
  assert str_to_lower("PYTHON")=="python"
  assert str_to_lower("12345!@#")=="12345!@#"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower() if s else ""