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