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);