R1:dd21761df9e8

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

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