Don't claim logged out users are automatically subscribed to un-owned objects

Summary: The PHID for logged out users is NULL, but so is the PHID for un-owned objects.

Test Plan: Browse a non-owned object (i.e. unassigned task) while logged out, notice "Automatically subscribed" before this commit.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12788