solidity中如何判断一个结构体为空？

pragma solidity ^0.6.0;
pragma experimental ABIEncoderV2;


contract Market  {

    struct Fruit {
        uint amount;
    }


    uint256[] public fruitId;
    mapping (uint256 => Fruit) private _fruits;

    function gettotal() public view returns (uint) {
        return fruitId.length;
    }


    function getFruit(uint256 _fruitId) public view returns (Fruit memory) {
        return _fruits[_fruitId];
    }

    function addFruit(uint256 _fruitId) public   {
        //require(_fruits[_fruitId]== Market.Fruit(0),"added"); 好像不能用0判断
        _fruits[_fruitId]    =  Fruit( 0);
        fruitId.push(_fruitId) ;
    }
}

添加一个新id的时候要判断一下这个fruit是否已经添加过，怎么判断这个id对应的结构体是不是空呢？