Raise an exception when called without any items

Calling `ShipmentPacker.pack` with an empty item list raises a `undefined method '[]' for nil:NilClass` exception. With this PR, we throw a `NoItemsProvided` error, which can be rescued.

Not sure if we should instead return an empty package list.

@csaunders any thoughts?

ref.: https://github.com/Shopify/shopify/issues/7572