Why is ‘nav_menu_item_args’ filter’s $item argument empty?

Filters (and actions) need to be declared with the number of arguments accepted by the callback; it is the fourth parameter of add_filter() (default 1):

add_filter( 'nav_menu_item_args', 'filter_nav_menu_item_args', 10, 3 );
function filter_nav_menu_item_args( $args, $item, $depth ) {

    print_r( $item );

    return $args;

}

See add_filter() for reference.