Introduce PhabricatorEmptyQueryException

Summary: It's dumb to execute a query which we know will return an empty result.

Test Plan: Looked at comment preview with "11", didn't see "1 = 0" in DarkConsole.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5177