R1:803a05082471

Fix an issue with creating new Conpherences

Summary: The participant list can sometimes be `null`, which fails when we try to `array_fuse()` it.

Test Plan: Created a new thread cleanly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11969