Fix C Task impl

Author: asvetlov

Modules/_asynciomodule.c

@@ -2003,15 +2003,15 @@ _asyncio.Task.__init__
     *
     loop: object = None
     name: object = None
-    context: object(c_default='NULL') = None
+    context: object = None
 
 A coroutine wrapped in a Future.
 [clinic start generated code]*/
 
 static int
 _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop,
                             PyObject *name, PyObject *context)
-/*[clinic end generated code: output=49ac96fe33d0e5c7 input=f45c94aefa9c235a]*/
+/*[clinic end generated code: output=49ac96fe33d0e5c7 input=924522490c8ce825]*/
 
 {
     if (future_init((FutureObj*)self, loop)) {
@@ -2030,7 +2030,7 @@ _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop,
         return -1;
     }
 
-    if (context == NULL) {
+    if (context == Py_None) {
         Py_XSETREF(self->task_context, PyContext_CopyCurrent());
         if (self->task_context == NULL) {
             return -1;