categoryPost.js

await Category.aggregate([
      {
        $lookup: {
          from: 'articles',
          as: 'article',
          let: { indicator_id: '$_id' },
          pipeline: [
            {
              $match: {
                $expr: { $eq: ['$category', '$$indicator_id'] }
              }
            },
            { $sort: { createdAt: -1 } },
            { $limit: 4 }
          ]
        }
      }, 
      {
        $match: {
          "article" : {$ne: []}
        }
      },
      {
        $sort: {
          createdAt: -1
        }
      }, 
      {
        $limit: 2
      }
    ]);

let categoryPost = await Category.aggregate([
      {
        $lookup: {
          from: 'articles',
          as: 'article',
          let: { indicator_id: '$_id' },
          pipeline: [
            {
              $match: {
                $expr: { $eq: ['$category', '$$indicator_id'] }
              }
            },
            { $sort: { createdAt: -1 } },
            { $limit: 4 }
          ]
        }
      },
      {
        $unwind: "article"
      }
    ]);

Try the above, some syntax might be off, but that is how to do it