woothemes/woocommerce — pull request #6352 by NickIvanter (2014-09-22 10:59:24+00:00)
Title: Allow WC_Order to be constructed w/o Order ID

This was allowed in WooCommerce 2.1 and some plugins rely on this behavior. Also, there seems to be no downside, as the parent class WC_Abstract_Order similarly doesn't require an Order ID at construction.