Why is the To field blank for Zendesk ticket emails sent to ticket CCs?
The address structure of the emails you are seeing is in accordance with the way CCs work with Zendesk, which is generally different from the way email clients operate.
Unlike standard email clients that send the same email to "To" and "CC" addresses, Zendesk processes notifications to CC addresses separately. In Zendesk replies to these notifications are not meant to update the requester/CC directly, rather they update the ticket, which in turn sends notifications based on present business rules.
For this reason the "To:" field on these notifications is always left blank. Otherwise the reply could go directly to the requester's address. Since the reply would also update the ticket, the requester would get two of the same notifications in emails generated within Zendesk.
Overall, in Zendesk the CC function allows you to tie others into the conversation, but the thread and notifications are intended to be lead within the ticket.
For more information on CC's in Zendesk, see How are CC's in Zendesk different from CC's in an email client?