Add migration to set default stock location on all shipments without stock location
user
```ruby
class SetDefaultStockLocationOnShipments < ActiveRecord::Migration
  def up
    if Spree::Shipment.where('stock_location_id IS NULL').count > 0
      location = DefaultStockLocation.find_or_create
      Spree::Shipment.where('stock_location_id IS NULL').update_all(stock_location_id: location.id)
    end
  end
end
```
assistant