Fix an incorrect filtering of groupless results from Maniphest "Group By" query

Summary:
Fixes T5081. This block enforces policies and prevents you from seeing groups associated with projects you can't see. However, it incorrectly removes tasks which end up with no group key. This can happen in two cases:

• The task isn't in a project.
• The task is in a project, but the query includes an "In All Projects: <that project>" constraint. In that case, we don't show the group becuase it would always contain every task.

Test Plan:
Replicated the setup in T5081, saw an "Ungrouped" group with "Task A":

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5081

Differential Revision: https://secure.phabricator.com/D9152