solidity智能合约如何判断mapping值为空

pragma solidity ^0.5.10;

contract TestCheckNullStruct{
    
    struct User{
        uint name;
        bool isUsed;
    }
    
    mapping(address => User) users;
    mapping(uint => address) addresses;
    
    function isExistEntry(address _addr) public view returns(bool){
        return users[_addr].isUsed;
    }
    
    function isExistAddress(uint _key) public view returns(bool){
        return addresses[_key] != address(0);
    }
}