gh-127182: Fix io.StringIO.__setstate__ crash when None is the first value

[sobolevn]

Change:

• We now check for explicit None value in __setstate__, it cannot be recieved normally, because __getstate__ never returns None, but we should not let a simple direct call to crash, when the fix is rather simple
• Why does it happen in the first place? Because we do the input validation with __init__ method:

  cpython/Modules/_io/stringio.c

  Line 898 in f7bb658

  if (_io_StringIO___init__((PyObject *)self, initarg, NULL) < 0) {

• But, None is an allowed input value, see

  cpython/Modules/_io/stringio.c

  Line 693 in f7bb658

  if (value && value != Py_None && !PyUnicode_Check(value)) {

• We did not check for None in __setstate__
• NULL cannot be passed to __setstate__, because it requires a tuple input

• Issue: Assertion failure from StringIO.__setstate__ #127182