Pass only user provided items details to PayPal API

PayPal API supports call with items details but does not require an
amount, quantity, description or url.

If the user has not specified these params, the XML code generated
should not output blank XML nodes.

When PayPal recives a quantity, they suppose you are also giving an
amount.

This is usefull for instance with carts that have a total price and
many items without any individual price.