R1:5b3b9b7182ae

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…