Fix EditEngine preview/draft for first comment on a task you didn't create

Summary: Ref T9132. See T9908#147038.

Test Plan:

• As user A, created a new task.
• As user B, started typing a comment on it (with no prior activity).
• Users A and B must be different.

Before patch: preview/draft don't work, trace in error log (see above).

After patch: preview/draft work.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14678