Should nonFungible attach/detach message have non-null token_index?

[egonspace]

Moved from https://github.com/line/link/issues/1368

Issue Description

It is not big deal but while I am look through messages from tx-result, I found token_index is null(missed) even though there is token-id contains token_index like below.

Attach message

{
  "height": "6927876",
  "tx_index": 0,
  "tx_hash": "BDAD6E8F43770BA409305061C0E3BC188CF860A746D0CA80BE33C5F75811082F",
  "msg_index": 0,
  "method": "collection/MsgAttach",
  "from": "tlink13dzju2wse9929wm7llma9lrt5vh378kd5gk85a",
  "to": null,
  "proxy": null,
  "msg": {
    "type": "collection/MsgAttach",
    "value": {
      "from": "tlink13dzju2wse9929wm7llma9lrt5vh378kd5gk85a",
      "contract_id": "36397e9e",
      "to_token_id": "1000000200000002",
      "token_id": "1000000200000001"
    }
  },
  "assets": [{
    "contract_id": "36397e9e",
    "asset_type": "ITEM",
    "fungible": false,
    "token_type": "10000002",
    "token_index": null
  }],
  "code": 23,
  "commit_time": "1631603268763"
}

Detach message

{
  "height": "7838130",
  "tx_index": 1,
  "tx_hash": "1A45DCBF923A83B54A8FE4F1AB09C08E34F176575D288349F11CE83ACDAE9312",
  "msg_index": 0,
  "method": "collection/MsgDetach",
  "from": "link1hqz6u2jz8d94wzrgkjxd8q5lezvgxw5yp3jqhf",
  "to": null,
  "proxy": null,
  "msg": {
    "type": "collection/MsgDetach",
    "value": {
      "from": "link1hqz6u2jz8d94wzrgkjxd8q5lezvgxw5yp3jqhf",
      "contract_id": "3336b76f",
      "token_id": "10000001000003b0"
    }
  },
  "assets": [{
    "contract_id": "3336b76f",
    "asset_type": "ITEM",
    "fungible": false,
    "token_type": "10000001",
    "token_index": null
  }],
  "code": 0,
  "commit_time": "1612877457156"
}

I guess attach and detach message should have token_index under assets, HDYT?