Fix transaction queries using withComments() for transactions with no comments

Summary:
See https://discourse.phabricator-community.org/t/daemons-tasks-crashing-in-a-loop-during-reindex/506/1. Some object types (for example, Passphrase Credentials) support indexing but not commenting.

Make withComments(...) work properly if the transaction type does not support comments.

Test Plan:
Indexed a credential (no comments) and a revision (comments) with bin/search index --trace ....

Before, credential fataled.

After, credetial succeeds, and skips the transaction query.

Before and after, the revision queries the transaction table.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18667