title: $limit and $skip as optional in MongoDB Aggregation pipeline
body: <p>I have a pipeline with <code>$facet</code> and using pagination inside it with <code>$skip</code> and <code>$limit</code>. I want to implement a condition when parameters are passed to skip and limit then only pagination should work else it should return me all the records.</p>
  <pre><code>Model.aggregate([
      {
        $facet: {
          comments: [
            {
              $match: { issueId: Types.ObjectId(issueId) },
            },
            {
              $skip: skip,
            },
            {
              $limit: parseInt(size, 10),
            },
            {
              $lookup: {
                from: 'users',
                localField: 'createdBy',
                foreignField: '_id',
                as: 'commentedBy',
              },
            },
          ]
        }
      }
  ]
  </code></pre>