[Bug](Datax)Fix bug that the dataxwriter will drop column when convert map to json (#13042)

* fix bug that when value is null，toJSONString will drop this key value.

docs/zh-CN/docs/ecosystem/datax.md

@@ -180,6 +180,7 @@ PROPERTIES (
                         "password": "xxx",
                         "postSql": [],
                         "preSql": [],
+                        "format":"csv|json",
                         "loadProps": {
                         },
                         "maxBatchRows" : 300000,
@@ -191,5 +192,6 @@ PROPERTIES (
     }
 }
 ```
+> 注意：```format``` 参数默认值：```csv```，如果想要使用```json```格式，需要在配置中指定```format``` 为```json```.
 
 4.执行datax任务，具体参考 [datax官网](https://github.com/alibaba/DataX/blob/master/userGuid.md)

extension/DataX/doriswriter/src/main/java/com/alibaba/datax/plugin/writer/doriswriter/DorisJsonCodec.java

@@ -19,6 +19,7 @@ package com.alibaba.datax.plugin.writer.doriswriter;
 
 import com.alibaba.datax.common.element.Record;
 import com.alibaba.fastjson.JSON;
+import com.alibaba.fastjson.serializer.SerializerFeature;
 
 import java.util.HashMap;
 import java.util.List;
@@ -45,6 +46,6 @@ public class DorisJsonCodec extends DorisCodec {
             rowMap.put(fieldName, this.convertColumn(row.getColumn(idx)));
             ++idx;
         }
-        return JSON.toJSONString(rowMap);
+        return JSON.toJSONString(rowMap, SerializerFeature.WriteMapNullValue);
     }
 }