Fix a URI construction in remarkup macro/meme rules

Summary: Ref T13250. Some of these parameters may be NULL, and `alter()` is no longer happy about that.

Test Plan: Ran daemon tasks that happened to render some memes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13250

Differential Revision: https://secure.phabricator.com/D20176

src/applications/macro/engine/PhabricatorMemeEngine.php

@@ -47,10 +47,13 @@ final class PhabricatorMemeEngine extends Phobject {
   }
 
   public function getGenerateURI() {
-    return id(new PhutilURI('/macro/meme/'))
-      ->alter('macro', $this->getTemplate())
-      ->alter('above', $this->getAboveText())
-      ->alter('below', $this->getBelowText());
+    $params = array(
+      'macro' => $this->getTemplate(),
+      'above' => $this->getAboveText(),
+      'below' => $this->getBelowText(),
+    );
+
+    return new PhutilURI('/macro/meme/', $params);
   }
 
   public function newAsset() {