protected JdbcSerializationConverter createNullableExternalConverter(LogicalType type) {
        return wrapIntoNullableExternalConverter(createExternalConverter(type), type);
    }
Create a nullable JDBC f{@link JdbcSerializationConverter} from given sql type.
inlong-sort/sort-flink/sort-flink-v1.13/sort-connectors/jdbc/src/main/java/org/apache/inlong/sort/jdbc/converter/AbstractJdbcRowConverter.java