Fix an issue with attempting to index comments on packages

Summary: See rPcd14194a329788d5fff6365bcade278fd18f3612 for a similar change. Implement getApplicationTransactionCommentObject() to return null explicitly.

Test Plan: Ran bin/search index --type ownerspackage, got indexing after change.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18715