Fix bad array index test in Differential package code

Summary: This needs an `isset()` for cases when authority and packages don't completely overlap.

Test Plan:
  - With a package set to trigger autoreview, created a revision.
  - Observed error log, saw no more error.
  - Saw package trigger autoreview properly.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16398

src/applications/differential/editor/DifferentialTransactionEditor.php

@@ -1533,7 +1533,7 @@ final class DifferentialTransactionEditor
 
     foreach ($packages as $key => $package) {
       $package_phid = $package->getPHID();
-      if ($authority[$package_phid]) {
+      if (isset($authority[$package_phid])) {
         unset($packages[$key]);
         continue;
       }