py/vm: Simplify handling of special-case STOP_ITERATION in yield from.

There's no need to have MP_OBJ_NULL a special case, the code can re-use
the MP_OBJ_STOP_ITERATION value to signal the special case and the VM can
detect this with only one check (for MP_OBJ_STOP_ITERATION).

py/runtime.c

@@ -1215,13 +1215,12 @@ mp_vm_return_kind_t mp_resume(mp_obj_t self_in, mp_obj_t send_value, mp_obj_t th
 
     if (type->iternext != NULL && send_value == mp_const_none) {
         mp_obj_t ret = type->iternext(self_in);
+        *ret_val = ret;
         if (ret != MP_OBJ_STOP_ITERATION) {
-            *ret_val = ret;
             return MP_VM_RETURN_YIELD;
         } else {
             // Emulate raise StopIteration()
             // Special case, handled in vm.c
-            *ret_val = MP_OBJ_NULL;
             return MP_VM_RETURN_NORMAL;
         }
     }

py/vm.c

@@ -1178,8 +1178,7 @@ yield:
                     } else if (ret_kind == MP_VM_RETURN_NORMAL) {
                         // Pop exhausted gen
                         sp--;
-                        // TODO: When ret_value can be MP_OBJ_NULL here??
-                        if (ret_value == MP_OBJ_NULL || ret_value == MP_OBJ_STOP_ITERATION) {
+                        if (ret_value == MP_OBJ_STOP_ITERATION) {
                             // Optimize StopIteration
                             // TODO: get StopIteration's value
                             PUSH(mp_const_none);