huobi websocket returns trade with direction of empty string, which side is this trade?

Question

Following huobi API, I subscribe to the trade.detail channel, and among one day data, I got one entry like this:

{
        "ch": "market.BTC-USDT.trade.detail",
        "ts": 1666158766936,
        "tick": {
            "id": 118671341528,
            "ts": 1666158766920,
            "data": [
                {
                    "amount": 2,
                    "quantity": 0.002,
                    "trade_turnover": 38.4776,
                    "ts": 1666158766920,
                    "id": 1186713415280000,
                    "price": 19238.8,
                    "direction": ""
                }
            ]
        }
    }

the direction field is empty string, don't know which side it is.