There’s a null
operator you can use for this type of query. See Field operators and data service Q&A
ICollection<DistributorLitiumProductInfoModel> prods = query
.Filter(filter => filter
.Bool(boolFilter => boolFilter.Must(boolFilterMust => boolFilterMust
.Field(ProductFieldNameConstants.ExpiredFromDistributors, "null", null))
))
.ToList()