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:

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