Fix constructing empty namedtuple

this change from micropython was not taken with the merge

py/objnamedtuple.c

@@ -115,9 +115,10 @@ mp_obj_t namedtuple_make_new(const mp_obj_type_t *type_in, size_t n_args, size_t
         #endif
     }
 
-    // Create a tuple and set the type to this namedtuple
-    mp_obj_tuple_t *tuple = MP_OBJ_TO_PTR(mp_obj_new_tuple(num_fields, NULL));
-    tuple->base.type = type_in;
+    // Create a namedtuple with explicit malloc. Calling mp_obj_new_tuple
+    // with num_fields=0 returns a read-only object.
+    mp_obj_tuple_t *tuple = mp_obj_malloc_var(mp_obj_tuple_t, mp_obj_t, num_fields, type_in);
+    tuple->len = num_fields;
 
     // Copy the positional args into the first slots of the namedtuple
     memcpy(&tuple->items[0], args, sizeof(mp_obj_t) * n_args);