When running a fulltext query with no query, enforce order by document creation date

Summary:
Fixes T11929. When running with a query, we no longer enforce an order on the subquery join to produce results more quickly when searching for common strings.

However, this means that empty queries (like those issued by "Close as Duplicate") don't order subquery results.

Restore a dateCreated order if there is no query text.

Test Plan: Artificially set limit to 10, still saw 10 most recent tasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11929

Differential Revision: https://secure.phabricator.com/D16960