title: filtering fields without entering null values
body: <p>collection:</p>
  
  <pre><code>{
  "_id" : ObjectId("5338ec2a5b5b71242a1c911c"),
  "people" : [ 
      {
          "name" : "Jhon"
      }, 
      {
          "age" : "30"
      }, 
      {
          "weight" : "80"
      }
  ]}
  </code></pre>
  
  <p>Query: db.tmp.aggregate({$project:{"people.name":1}})</p>
  
  <p>In the issue:</p>
  
  <pre><code>{
  "result" : [ 
      {
          "_id" : ObjectId("5338ec2a5b5b71242a1c911c"),
          "people" : [ 
              {
                  "name" : "Jhon"
              }, 
              {}, 
              {}
          ]
      }
  ],
  "ok" : 1}
  </code></pre>
  
  <p>How to display only the "name" field blank without {}?</p>