Conpherence - fix bug with possible null value

Summary: Fixes T7675. epriestley caught this in code review and I didn't implement it all the way

Test Plan: logicypoo

Reviewers: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7675

Differential Revision: https://secure.phabricator.com/D12181

src/applications/conpherence/view/ConpherenceThreadListView.php

@@ -35,7 +35,7 @@ final class ConpherenceThreadListView extends AphrontView {
 
     $grouped = mgroup($this->threads, 'getIsRoom');
     $rooms = idx($grouped, true, array());
-    $rooms = array_slice($grouped[true], 0, 5);
+    $rooms = array_slice($rooms, 0, 5);
 
     $policies = array();
     foreach ($rooms as $room) {