Improve performance of empty search

Summary:
Searching for empty query kills us because whole `search_document` table is read in this case.

This diff adds an index just for this query.

Other solution would be to disable searching for empty string. But it can be actually useful (listing newest documents of any type).

Test Plan:
  lang=sql
  EXPLAIN SELECT document.phid, document.documentType, document.documentTitle, document.documentCreated
  FROM `search_document` document
  GROUP BY document.phid
  ORDER BY documentCreated DESC
  LIMIT 0, 21;

Reviewers: epriestley

Reviewed By: epriestley

CC: jungejason, aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2277

resources/sql/patches/134.emptysearch.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `phabricator_search`.`search_document`
+  ADD INDEX `documentCreated` (`documentCreated`);