fix grouping in Experiments.find selector

Equivalent JavaScript before this change:

    batchId: {
      $exists: false,
      startTime: {
        $exists: true
      }
    }

After:

    batchId: {
      $exists: false,
    }
    startTime: {
      $exists: true
    }

Found this with MongoDB 2.6.5 (more strict than MongoDB 2.4.x).
Be forwards-compatible with MongoDB 2.6.5

$set: {} is no longer allowed.