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