update tbl set field1 = isnull(?, field1), field2 = isnull(?, field2) where id = ?

use DoctrineDBALDriverManager;

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

$connectionOptions = array_merge(
    ['driverClass' => 'DoctrineDBALDriverPDOSqlsrvDriver'],
    ['host' => '127.0.0.1', 'port' => 1433, 'dbname' => 'users', 'user' => '', 'password' => '']
);

try {
    $conn = DriverManager::getConnection($connectionOptions);
} catch (Exception $e) {
    die($e->getMessage());
}

$userId = 1;
$nick = 'test';

try {
    $result = $conn->executeUpdate(
        'UPDATE users.users SET user_nick = isnull(:nick, user_nick) WHERE user_id = :userId',
        ['nick' => $nick, 'userId' => $userId],
        ['nick' => PDO::NULL_EMPTY_STRING, 'userId' => PDO::PARAM_INT]
    );
} catch (Exception $e) {
    die($e->getMessage());
}