py/map: Clear value when re-using slot with ordered dictionaries.

To adhere to the contract of mp_map_lookup, namely:

    MP_MAP_LOOKUP_ADD_IF_NOT_FOUND behaviour:
    - returns slot, with key non-null and value=MP_OBJ_NULL if it was added

py/map.c

@@ -221,6 +221,7 @@ mp_map_elem_t *MICROPY_WRAP_MP_MAP_LOOKUP(mp_map_lookup)(mp_map_t * map, mp_obj_
         }
         mp_map_elem_t *elem = map->table + map->used++;
         elem->key = index;
+        elem->value = MP_OBJ_NULL;
         if (!mp_obj_is_qstr(index)) {
             map->all_keys_are_qstrs = 0;
         }