Fix #31856: movieclips.load(filepath=None) or value crash Blender

Disallow running PyUnicode_EncodeFSDefault for None type which seems
to be an issue on Windows.

source/blender/python/generic/py_capi_utils.c

@@ -384,7 +384,10 @@ const char *PyC_UnicodeAsByte(PyObject *py_str, PyObject **coerce)
 	else {
 		PyErr_Clear();
 
-		if (PyBytes_Check(py_str)) {
+		if (py_str == Py_None) {
+			return NULL;
+		}
+		else if (PyBytes_Check(py_str)) {
 			return PyBytes_AS_STRING(py_str);
 		}
 		else if ((*coerce = PyUnicode_EncodeFSDefault(py_str))) {