ensure /_api/me/user-group-relations to return valid data

lib/models/user-group-relation.js

@@ -99,7 +99,12 @@ class UserGroupRelation {
     return this
       .find({ relatedUser: user.id })
       .populate('relatedGroup')
-      .exec();
+      // filter documents only relatedGroup is not null
+      .then(userGroupRelations => {
+        return userGroupRelations.filter(relation => {
+          return relation.relatedGroup != null;
+        });
+      });
   }
 
   /**