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