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

src/applications/transactions/query/PhabricatorApplicationTransactionQuery.php

@@ -203,6 +203,18 @@ abstract class PhabricatorApplicationTransactionQuery
       $xaction = $this->getTemplateApplicationTransaction();
       $comment = $xaction->getApplicationTransactionCommentObject();
 
+      // Not every transaction type has comments, so we may be able to
+      // implement this constraint trivially.
+
+      if (!$comment) {
+        if ($this->withComments) {
+          throw new PhabricatorEmptyQueryException();
+        } else {
+          // If we're querying for transactions with no comments and the
+          // transaction type does not support comments, we don't need to
+          // do anything.
+        }
+      } else {
         if ($this->withComments) {
           $joins[] = qsprintf(
             $conn,
@@ -215,6 +227,7 @@ abstract class PhabricatorApplicationTransactionQuery
             $comment->getTableName());
         }
       }
+    }
 
     return $joins;
   }