Return 404 instead of undefined variable error when trying to edit a non-existent form

Summary: E.g. https://phab-01.wmflabs.org/transactions/editengine/transactions.editengine.config/view/13/

Test Plan:
* Go to /transactions/editengine/transactions.editengine.config/view/1000000/
* Observe error
* Apply patch
* Observe 404

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D15307

src/applications/transactions/controller/PhabricatorEditEngineController.php

@@ -70,6 +70,8 @@ abstract class PhabricatorEditEngineController
       ->executeOne();
     if ($config) {
       $engine = $config->getEngine();
+    } else {
+      return null;
     }
 
     if (!$engine->isEngineConfigurable()) {