Fix "Any Owner" and "No Owners" searches in Maniphest

Summary:
See https://discourse.phabricator-community.org/t/maniphest-home-page-crash-after-d19417/1445/3. These special-token-only searches currently end up populating an empty ownerPHIDs, which fatals after the stricter check in D19417.

Make the fatal on withConstraint(array()) explicit and only set the PHID constraint if we have some PHIDs left.

Test Plan: Searched for "No Owner", "Any Owner", an actual owner, "No Owner + actual user".

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19440