presto-main/src/main/java/io/prestosql/cost/ScalarStatsCalculator.java/ScalarStatsCalculator/nullStatsEstimate
class ScalarStatsCalculator:    private static SymbolStatsEstimate nullStatsEstimate()
    {
        return SymbolStatsEstimate.builder()
                .setDistinctValuesCount(0)
                .setNullsFraction(1)
                .build();
    }