title: robo 3t - Aggregate query on array fails
body: <p><strong>Setup:</strong>
  Documents in parent collection have a field children with child identifiers. Documents in the child collection have the identifier in a customId field not _id. A single identifier from the 'children' field may match multiple children.</p>
  
  <p><strong>What's wrong:</strong>
  This query</p>
  
  <pre><code>db.getCollection('parent').aggregate({$lookup: {from: "child", localField: "children", foreignField:"customId", as:"joinedChildren"}})
  </code></pre>
  
  <p>runs correctly in mongo shell but causes</p>
  
  <pre><code>Error:
  TypeError: pipeline[(pipeline.length - 1)] is undefined :
  DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1292:9
  DBCollection.prototype.aggregate@:1:355
  @(shell):1:1
  </code></pre>
  
  <p>in robo3T.</p>