R1:4da88848bed8

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…