[hotfix][hdfs]when writing to hdfs, null value is set to '\N' (#1408)

[hotfix][hdfs]when writing to hdfs, null value is set to '\N' for hive specification so that hdfs connector can read from the output

(cherry picked from commit a924b98)

chunjun-connectors/chunjun-connector-hdfs/src/main/java/com/dtstack/chunjun/connector/hdfs/converter/HdfsTextColumnConverter.java

@@ -130,7 +130,7 @@ protected ISerializationConverter<String[]> wrapIntoNullableExternalConverter(
             ISerializationConverter serializationConverter, String type) {
         return (rowData, index, data) -> {
             if (rowData == null || rowData.isNullAt(index)) {
-                data[index] = null;
+                data[index] = "\\N";
             } else {
                 serializationConverter.serialize(rowData, index, data);
             }