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Running this query on any collection:

{non_existing_field: {$ne: null}}

will return no objects, since

The \{ item : null } query matches documents that either contain the item field whose value is null or that do not contain the item field

However, running

{$expr: {$and: [
              {
              $ne: [
                non_existing_field, null
              ]
            }
  ]}}

Returns all the fields from the collection- seems like it always returns true.