Custom Static Links For Specific Menu Right Before/Next The wp_nav_menu Function now working

If you will not pass that how many arguments you need in your callback function then add_filter will provide you only one and first argument.

Because default arguments for add_filter are

add_filter ( string $tag, callable $function_to_add, int $priority = 10, int $accepted_args = 1 );

$accepted_args is 1 if you do not pass it.

Thus $args is always blank array in your callback function.

Change

add_filter('wp_nav_menu_items',do_wp_nav_menu_items);

to

add_filter('wp_nav_menu_items', 'do_wp_nav_menu_items', 10, 2);