Fix some CC handling in Maniphest

Summary:
Fixes T6932. Fixes some issues from D11303.

• When claiming a task, if it was previously unassigned, we would try to CC null.
• When claiming a task, if the current owner was already CC'd, the viewer would incorrectly be warned about all subscribers being CC'd.

Test Plan:

• Claimed an unclaimed task.
• Claimed a task with owner CC'd.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T6932

Differential Revision: https://secure.phabricator.com/D11336