Search for nil values on Atlas Search filter

[ppascualv]

I have a pipeline that start with:

{
        '$search': {
          index: 'next_task',
          compound: {
            filter: [
              { equals: { path: 'account_id', value: account.id } },
              { in: { path: 'playbook_id', value: user.playbook_ids } },
              { equals: { path: 'status', value: 'created' } },
              { equals: { path: 'transferable', value: false } },
              { equals: { path: 'assignee_id', value: nil } },
              { range: { path: 'active_on', lte: Time.zone.now } }
            ].concat(predictive_call_query(predictive_call), user_skills),
            mustNot: [
              { equals: { path: 'internal_identifier', value: 'incoming_call' } }
            ]
          },
          sort: { priority: 1, attempts: 1, identifier_priority: 1 }
        }
      },
      

The issue happens because { equals: { path: 'assignee_id', value: nil } } is not really filterig nil values.

Does anyone know how can I use nil to filter? According to Mongo community is already implemented, and it works on Atlas Search tester, seems something related to the library

https://www.mongodb.com/docs/atlas/atlas-search/equals/