R1:bed9ce2d1897

Make PeopleQuery throw, not select everything, when handed empty array

Summary: Make `->withPHIDs(array())` throw on this query instead of selecting everything.

Test Plan: Poked around.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9210