title: "unwinding": { "preserveNullAndEmptyArrays": false } not working in v3.4 aggregate $lookup
body: <p>i am trying to run a aggregation pipeline with lookup stage . 
  the pipline is taking to long so i decided to optimize the pipline in the unwind stage by removing documents with empty arrays.  </p>
  
  <p>the docs on the mongo site suggest that this should work but in v3.4.</p>
  
  <p>my lookup stage looks like this</p>
  
  <pre><code>    {
          "$lookup": {
              "from": "abc",
              "localField": "abc_id",
              "foreignField": "abc_id",
              "as": "abc_info",
              "unwinding": { "preserveNullAndEmptyArrays": false }
          }
       }
  </code></pre>
  
  <p>im getting the errror </p>
  
  <pre><code>    "arguments to $lookup must be strings, unwinding: { 
      preserveNullAndEmptyArrays: false } is type object"
  </code></pre>