Expected to get a 64-bit private key, but actually got 63 bits

[bai-xuan]

Bug_title

A clear and concise description of what the issue is.
Expected to get a 64-bit private key, but actually got 63 bits

Steps To Reproduce

Describe the exact steps or scenario to reproduce the issue.
`
import org.web3j.crypto.Bip32ECKeyPair;
import org.web3j.crypto.Credentials;
import org.web3j.crypto.MnemonicUtils;

import java.security.SecureRandom;

public static void main(String[] args) {
SecureRandom secureRandom = new SecureRandom();

    while (true) {
        byte[] initialEntropy = new byte[16];
        secureRandom.nextBytes(initialEntropy);
        String mnemonic = MnemonicUtils.generateMnemonic(initialEntropy);
        String password = "";
        byte[] seed = MnemonicUtils.generateSeed(mnemonic, password);
        Bip32ECKeyPair bip32ECKeyPair = Bip32ECKeyPair.generateKeyPair(seed);
        Credentials credentials = Credentials.create(bip32ECKeyPair);
        String privateKeyStr = credentials.getEcKeyPair().getPrivateKey().toString(16);
        String address = credentials.getAddress();
        if (privateKeyStr.length() != 64) {
            System.out.println("The private key length is incorrect");
            System.out.println("address: " + address);
            System.out.println("privateKeyStr: " + privateKeyStr);
            System.out.println("privateKeyStr length: " + privateKeyStr.length());
            System.out.println("mnemonic: " + mnemonic);
            break;
        }
    }
}`

Expected behavior

A clear and concise description of what you expected to happen.

Expected to get a 64-bit private key, but actually got 63 bits

Environment

Describe the environment in which the issue occurs

• Web3j version :4.12.3
• Java or Android version :openjdk-17.0.11+9
• Operating System :win11