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