bpo-43687: use unicode_state empty string before unicode_init. without define WITH_DOC_STRINGS

[JunyiXie]

without define WITH_DOC_STRINGS. use unicode_state empty string before _PyUnicode_Init.

PyType_Ready call PyUnicode_FromString, if doc string "", cause crash.

unicode_get_empty() must not be called before _PyUnicode_Init() or after _PyUnicode_Fini()

PyType_Ready

const char *old_doc = _PyType_DocWithoutSignature(type->tp_name,type->tp_doc);
PyObject *doc = PyUnicode_FromString(old_doc);

https://bugs.python.org/issue43687

[JunyiXie]

This commit should not need to add news.

[JunyiXie]

@vstinner Please review this pr? thanks.I found this crash, I am not sure that my fix are reasonable.

[vstinner]

This change removes my micro-optimization:

static int
unicode_create_empty_string_singleton(struct _Py_unicode_state *state)
{
    // Use size=1 rather than size=0, so PyUnicode_New(0, maxchar) can be
    // optimized to always use state->empty_string without having to check if
    // it is NULL or not.

I wrote PR #25147 which reorganizes Py_Initialize() to fix https://bugs.python.org/issue43687 without losing the optimization.

[vstinner]

I merged PR #25147 instead, but thanks anyway.