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("PYTHON")=="python"
  assert str_to_lower("123abc")=="123abc"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower() if s else ""