Fix an error in Diffusion when the Owners application is uninstalled

Summary:
See <https://discourse.phabricator-community.org/t/undefined-view-when-owners-is-uninstalled/451>.

When Owners is not installed, Diffusion can fatal with a bad `$view`.

Test Plan:
  - Uninstall Owners.
  - View the content of any file in Diffusion.
  - Before: fatal on `$view` undefined.
  - After: Valid page with no owners information.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18629

src/applications/diffusion/controller/DiffusionBrowseController.php

@@ -799,12 +799,17 @@ final class DiffusionBrowseController extends DiffusionController {
   }
 
   private function buildOwnersList(DiffusionRequest $drequest) {
-
     $viewer = $this->getViewer();
+
+    $have_owners = PhabricatorApplication::isClassInstalledForViewer(
+      'PhabricatorOwnersApplication',
+      $viewer);
+    if (!$have_owners) {
+      return null;
+    }
+
     $repository = $drequest->getRepository();
 
-    $owners = 'PhabricatorOwnersApplication';
-    if (PhabricatorApplication::isClassInstalled($owners)) {
     $package_query = id(new PhabricatorOwnersPackageQuery())
       ->setViewer($viewer)
       ->withStatuses(array(PhabricatorOwnersPackage::STATUS_ACTIVE))
@@ -866,7 +871,6 @@ final class DiffusionBrowseController extends DiffusionController {
       ->setBackground(PHUIObjectBoxView::BLUE_PROPERTY)
       ->addClass('diffusion-mobile-view')
       ->setObjectList($ownership);
-    }
 
     return $view;
   }