Don't try to emit project board update events if there are no projects to update

Summary: Ref T4900. We may execute a bad query here if the task has no projects at all.

Test Plan: Edited a task with no new or old projects. Instead of an exception, things worked.

Maniphest Tasks: T4900

Differential Revision: https://secure.phabricator.com/D20689

src/applications/maniphest/editor/ManiphestTransactionEditor.php

@@ -879,6 +879,7 @@ final class ManiphestTransactionEditor
       $project_phids = array_fuse($old_phids) + array_fuse($new_phids);
       $project_phids = array_keys($project_phids);
 
+      if ($project_phids) {
         $projects = id(new PhabricatorProjectQuery())
           ->setViewer(PhabricatorUser::getOmnipotentUser())
           ->withPHIDs($project_phids)
@@ -909,6 +910,7 @@ final class ManiphestTransactionEditor
           PhabricatorNotificationClient::tryToPostMessage($data);
         }
       }
+    }
 
     return $xactions;
   }