Make "arc tasks --unassigned" actually find unassigned tasks

Summary:
Ref T3201. D6009 added this flag, but I don't think it actually works as advertised? We either need a Conduit patch to interpret `null` as `upforgrabs` (which seems 100% reasonable to me) or this (which is kind of nasty).

@garoevans, was there a Phabricator-side diff for the conduit part that just got dropped? I'll probably replace this with that if not, but figured I'd check before I poke anything.

Test Plan: Ran `arc diff --unassigned`

Reviewers: garoevans

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T3201

Differential Revision: https://secure.phabricator.com/D6062