Don't compute a nonsensical assigned task count for logged-out users

Summary:
When a logged-out user views the home page, we currently compute a meaningless "assigned tasks" count which just counts every open task. Don't do this.

Ideally `ManiphestTaskQuery` should get some tightening up too (that is, `withOwners(array(null))` should not select every task), but that might affect other stuff and the performance implications of counting every open task are affecting WMF, so just fix the immediate issue for now.

Test Plan:
  - Viewed homepage as a logged out user, no assigned task count.
  - Viewed homepage as a logged-in user, still saw assigned task count.

Reviewers: btrahan, chad, chasemp

Reviewed By: chasemp

Subscribers: 20after4, epriestley

Differential Revision: https://secure.phabricator.com/D10876