SelectFilter Add Null as an option

I have a select filter by manager:

SelectFilter::make('manager_id')
->options(Manager::pluck('name', 'id')->toArray())

SelectFilter::make('manager_id')
->options(Manager::pluck('name', 'id')->toArray())

How do I add a Null Only as another filter Option?

Currently I am using a seperate filter for Null Only

Filter::make('null_manager')
->query(fn(Builder $query) =>
       $query
           ->whereNull('manager_id') )

Filter::make('null_manager')
->query(fn(Builder $query) =>
       $query
           ->whereNull('manager_id') )

How do I combine the Null Only with the manager select?