presto-local-file/src/main/java/io/prestosql/plugin/localfile/LocalFileRecordCursor.java/LocalFileRecordCursor/isNull
class LocalFileRecordCursor:    @Override
    public boolean isNull(int field)
    {
        checkArgument(field < columns.size(), "Invalid field index");
        String fieldValue = getFieldValue(field);
        return "null".equals(fieldValue) || Strings.isNullOrEmpty(fieldValue);
    }