Fix querying for transactions over "transaction.search" when the object does not support comments

Summary: See PHI725. Ref T13151. We currently try to load comments unconditionally, but not all objects (like projects) have comments. Only try to load comments if an object actually has comments.

Test Plan: Queried for an object with no comments, like project #masonry, via transaction.search.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19507