Fix "To: Unknown Object" on outbound Maniphest mail with no owner

Summary: Fixes T7778. This was likely caused by removing an `array_filter()` somewhere in the course of T7731, but I'd rather have the code be more correct.

Test Plan:
Sent mail on a task with no owner.

  - Before patch: unknown recipient.
  - After patch: expected recipients.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T7778

Differential Revision: https://secure.phabricator.com/D12320

src/applications/maniphest/editor/ManiphestTransactionEditor.php

@@ -386,10 +386,14 @@ final class ManiphestTransactionEditor
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
-    return array(
-      $object->getOwnerPHID(),
-      $this->getActingAsPHID(),
-    );
+    $phids = array();
+
+    if ($object->getOwnerPHID()) {
+      $phids[] = $object->getOwnerPHID();
+    }
+    $phids[] = $this->getActingAsPHID();
+
+    return $phids;
   }
 
   public function getMailTagsMap() {