R1:10a4b05ecba6

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".