When Spree::ActiveShipping::Config[:shipper_number] is "", an empty ShipperNumber node is added to the request, resulting in a "Missing/Invalid Shipper/ShipperNumber" error response from UPS.

By adding .presence, we can have it treat a "" value as nil so that it will skip adding the
ShipperNumber node when shipper_number is "" instead of treating "" as a truthy value.

Fixes #57