title: Spring Data - MongoDb aggregation $ifNull
body: <pre><code>db.collection.aggregate([
      {$match : { name : "name" } },
      {$project: {
          name: 1,
          sent: { 
              $size: {
                  "$ifNull": [ "$audience", [] ]
              } 
          }
      }
  }]);
  </code></pre>
  
  <p>How can I do the above mongo aggregation with Spring data?</p>