Fix calling getSalt() on possibly null value in AuthenticationMiddleware
Fix calling getSalt() on possibly null value in AuthenticationMiddleware

This error would probably never have happened in real life, but here is a synthetic reproducer:

// test_auth.php
<?php

// run php -S localhost:8000 test_auth.php,
// then open the http://localhost:8000 page and refresh it

declare(strict_types=1);

use Tarantool\Client\Client;
use Tarantool\Client\Middleware\AuthenticationMiddleware;

require __DIR__.'/vendor/autoload.php';

$authMiddleware = new AuthenticationMiddleware('guest');

$client = Client::fromDsn('tcp://localhost:3301');
$client = $client->withMiddleware($authMiddleware);
$client->ping();

echo "refresh the page to see an error";

$client2 = Client::fromDsn('tcp://localhost:3301?persistent=true');
$client2 = $client2->withMiddleware($authMiddleware);
$client2->ping();

tarantool-php/client,tarantool-php/client