Fix exception accessing a non-existing EditEngine

Summary:
Trying to access a non-existing edit engine "foo" via the URI `/transactions/editengine/foo/` throws an unhandled exception.
`PhabricatorEditEngine::getByKey($viewer, $engine_key)` is null as `$engine_key` is non-existing `foo`.
Thus avoid calling `setViewer()` on `null` by splitting into a separate step after a null check.
When null, show a proper 404 error instead.

```
EXCEPTION: (Error) Call to a member function setViewer() on null at [<phorge>/src/applications/transactions/controller/PhabricatorEditEngineConfigurationListController.php:17]
```

Closes T15793

Test Plan:
While logged in and with sufficient permissions,
* go to non-existing http://phorge.localhost/transactions/editengine/whatever/ before and after applying patch,
* go to existing http://phorge.localhost/transactions/editengine/maniphest.task/ to see that existing stuff still works.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15793

Differential Revision: https://we.phorge.it/D25597

src/applications/transactions/controller/PhabricatorEditEngineConfigurationListController.php

@@ -13,12 +13,11 @@ final class PhabricatorEditEngineConfigurationListController
     $engine_key = $request->getURIData('engineKey');
     $this->setEngineKey($engine_key);
 
-    $engine = PhabricatorEditEngine::getByKey($viewer, $engine_key)
-      ->setViewer($viewer);
-
-    if (!$engine->isEngineConfigurable()) {
+    $engine = PhabricatorEditEngine::getByKey($viewer, $engine_key);
+    if (!$engine || !$engine->isEngineConfigurable()) {
       return new Aphront404Response();
     }
+    $engine->setViewer($viewer);
 
     $items = array();
     $items[] = id(new PHUIListItemView())