$allowed = (array) get_option('rnd_cm_export_categories', []);

$args = [
    'post_type'      => 'post',
    'posts_per_page' => $this->batch,
    'offset'         => $this->offset,
    'post_status'    => 'publish',
];

if (!empty($allowed)) {
    $args['category__in'] = $allowed;
}

$q = new \WP_Query($args);