show user based on user_meta

you can create conditionnal arguments like that :

$user_args = [
    'orderby' => 'display_name',
    'order' => 'asc',
    'meta_query' => array(
        array(
            'key' => 'verifier_assign_country',
            'value' => $countryCode,
            'compare' => '='
        ),
    )
];

if ("" !== $univCode) {

    $user_args["meta_query"][] = array(
        'key' => 'verifier_assign_univ',
        'value' => $univCode,
        'compare' => 'LIKE'
    );

}

$users = get_users($user_args);