When I try to add user meta to empty array as: a:0:{} it’s replaced into database as s:6:"a:0:{}";

I just need to save it as I written as a:0:{}

this is my code:

$sassada = get_user_meta($current_user, 'custom_system');   
if(empty($sassada)){
    add_user_meta($sassada, 'custom_system', 'a:0:{}');
}