Como excluir valores nulos da consulta de preenchimento do Mongoose

var results =await dbo.collection('mediators').aggregate([
     { $lookup:
         {
           from: 'venues',
           localField: 'venue',
           foreignField: '_id',
           as: 'venue'
         }
      },
        $match:{$and:[{"venue.venue_type":req.query.venue_type} , {"venue.capacity":{$gte:parseInt(req.query.capacitylb) , $lte:parseInt(req.query.capacityub)}}]}
      },{
        $lookup:
         {
           from: 'users',
           localField: 'user',
           foreignField: '_id',
           as: 'user'
         }
      },{
        $lookup:
        {
           from: 'professionals',
           localField: 'professional',
           foreignField: '_id',
           as: 'professional'
        }
      },{
        $lookup:
        {
          from:'availabilities',
          localField: 'availability',
          foreignField: '_id',
          as: 'availability'
        }
      },{
        $unwind: '$availability'
      },{
        $match:{$and:[{"availability.start":{$lte:new Date(req.query.dateFrom)}},{"availability.end":{$gte:new Date(req.query.dateTo)}}]}
      },{
        $lookup:
        {
          from:'locations',
          localField: 'location',
          foreignField: '_id',
          as: 'location'
        }
      },{
        $project:{
          "_id":1,
          "email":"$user.email",
          "organization_name":"$user.organization_name",
          "website":"$user.website",
          "profile_pic_hash":"$user.profile_pic_hash",
          "bio_hash":"$user.bio_hash",
          "venue_type":"$venue.venue_type",
          "capacity":"$venue.capacity",
          "flat_fee":"$professional.flat_fee",
          "per_participant_fee":"$professional.per_participant_fee",
          "hourly_fee":"$professional.hourly_fee",
          "start_date":"$availability.start",
          "end_date":"$availability.end",
          "distance":"$dist.calculated",
          "location":"$location.location.coordinates",
          "country":"$location.country",
          "city":"$location.city",
          "street":"$location.street",
          "number":"$location.number",
          "zip":"$location.zip"}}