solidity default value for interface instance

pragma solidity 0.6.6;

interface IERC20{
    function name() external view returns (string memory);
}

contract DefaultValue{
    IERC20 public a_erc20; // default value 0x0000000000000000000000000000000000000000
    mapping(address => IERC20) public erc20s; // default value for a non-exist key address(nonexistKey) = address(0)

    function addErc20(address addr) public{
        erc20s[addr] = IERC20(addr);
    }

    function isExist(address addr) public view returns (bool){
        if (address(erc20s[addr]) == address(0)){
            return false;
        }
        return true;
    }
}