Fix some error log issues with uninitialized commit/revision lists

Summary:
Fixes T12679. Reproduction steps appear to be:

  - As a logged-out user, view revision list or commit list.
  - Enable bucketing by action required.
  - Before patch: `foreach (null as ...)` causes error spew.
  - After patch: `foreach (array() as ...)` works great.

Test Plan:
  - Reproduced issue by following steps above in Differential (revisions) and Diffusion (audits/commits).
  - After patches, no more errors in the log.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12679

Differential Revision: https://secure.phabricator.com/D17872

src/applications/audit/view/PhabricatorAuditListView.php

@@ -2,7 +2,7 @@
 
 final class PhabricatorAuditListView extends AphrontView {
 
-  private $commits;
+  private $commits = array();
   private $header;
   private $showDrafts;
   private $noDataString;

src/applications/differential/view/DifferentialRevisionListView.php

@@ -5,7 +5,7 @@
  */
 final class DifferentialRevisionListView extends AphrontView {
 
-  private $revisions;
+  private $revisions = array();
   private $handles;
   private $header;
   private $noDataString;