Skip Ferret fulltext columns in "ORDER BY" if there's no fulltext query

Summary:
Ref T13091. If you "Order By: Relevance" but don't actually specify a query, we currently raise a bare exception.

This operation is sort of silly/pointless, but it seems like it's probably best to just return the results for the other constraints in the fallback order (usually, by ID). Alternatively, we could raise a non-bare exception here ("You need to provide a fulltext query to order by relevance.")

Test Plan: Queried tasks by relevance with no actual query text.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13091

Differential Revision: https://secure.phabricator.com/D20296