Fix Maniphest fatal if attached tasks are not an array

Summary: Fixes T3678. I think some very old rows may have a junk value here. This will be obsoleted by ApplicationTransactions and other modernization, most likely, so just fix it locally.

Test Plan: looked at a task

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3678

Differential Revision: https://secure.phabricator.com/D6677

src/applications/maniphest/controller/ManiphestTaskDetailController.php

@@ -566,6 +566,10 @@ final class ManiphestTaskDetailController extends ManiphestController {
     }
 
     $attached = $task->getAttached();
+    if (!is_array($attached)) {
+      $attached = array();
+    }
+
     $file_infos = idx($attached, PhabricatorFilePHIDTypeFile::TYPECONST);
     if ($file_infos) {
       $file_phids = array_keys($file_infos);