Don't return any results for viewerprojects() if the viewer is in no projects

Summary:
Fixes T10135. When the viewer is a member of no projects, specify the constraint type as a new "EMPTY" type.

When a query has an "EMPTY" constraint, fail fast with no results.

Test Plan:

• Viewed a viewerprojects() query result set as a user in no projects.
  • Before patch: got a lot of hits. After patch: no hits.
• Viewed a normal result set, no changes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10135

Differential Revision: https://secure.phabricator.com/D15003