An Extra Comma Appears When Using A Coupon That Gives Free Shipping

There's an annoying comma when you're using a Free Shipping coupon without a price discount. It looks fine when there is a discount but not so much when it's just free shipping. I made sure that any empty array elements are removed so there's no comma.

![checkout___wc_2_1_x](https://f.cloud.github.com/assets/1065372/2453982/b6c8b598-aee4-11e3-8641-02291352418d.png)

Originally from: https://woothemes.zendesk.com/agent/#/tickets/158290