R1:cd14194a3297

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…