Solidity sha256 returns "0x"

Question

I have a simple smart contract as below. Using go-ethereum-1.8.6 , when i execute contract.sha.call() it returns the correct hash value. However, contract.getSha256.call() it returns 0x, did I do something wrong?

pragma solidity ^0.4.23;

/* Online compiler https://remix.ethereum.org */

contract test{

    address public owner;

    constructor() public {
        owner = msg.sender;
    }

    function getOwner() external view returns (address) {
        return owner;
    }

    function getSha256() external pure returns (bytes32) {

        bytes32 hash = sha256("12345678");

        return (hash);
    }

    function sha() external pure returns (bytes32) {

        bytes32 hash = sha3("12345678");

        return (hash);
    }

}