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

src/applications/project/typeahead/PhabricatorProjectLogicalViewerDatasource.php

@@ -63,10 +63,16 @@ final class PhabricatorProjectLogicalViewerDatasource
     $phids = mpull($projects, 'getPHID');
 
     $results = array();
-    foreach ($phids as $phid) {
+    if ($phids) {
+      foreach ($phids as $phid) {
+        $results[] = new PhabricatorQueryConstraint(
+          PhabricatorQueryConstraint::OPERATOR_OR,
+          $phid);
+      }
+    } else {
       $results[] = new PhabricatorQueryConstraint(
-        PhabricatorQueryConstraint::OPERATOR_OR,
-        $phid);
+        PhabricatorQueryConstraint::OPERATOR_EMPTY,
+        null);
     }
 
     return $results;