$wpdb->wp_users returning empty value for

First of all why you would need custom query for this basic functionality to get users, when WordPress has inbuilt function get_users().

Anyway for custom query, table name is defined as

function custom_func(){
 global $wpdb;
 $getTest = "SELECT * FROM $wpdb->users LIMIT 1";
 $arrayReturned = $wpdb->get_results($wpdb->prepare($getTest));
}

It’s $wpdb->users or $wpdb->post and not $wpdb->wp_users, what you use for a table name, in custom queries.

Also I’ve used the double quotes ” “ for query and not single quotes ‘ ‘, you can read here