[Bug] Partial column update does not work on routine load

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

3.1

### What's Wrong?

created routine load according to this: https://doris.apache.org/docs/dev/data-operate/update/update-of-unique-model

CREATE TABLE test_user (
id INT,
name VARCHAR(255),
nick_name VARCHAR(255),
age INT
)
ENGINE = OLAP
UNIQUE KEY (id)
DISTRIBUTED BY HASH(id) BUCKETS 10
PROPERTIES (
"replication_num" = "1",
"enable_unique_key_merge_on_write" = "true"
);

CREATE ROUTINE LOAD test_user_routine_load ON test_user
COLUMNS(id, name, nick_name, age)
PROPERTIES
(
"format" = "json",
"max_batch_interval" = "10",
"max_batch_rows" = "200000",
"max_batch_size" = "104857600",
"strict_mode" = "false",
"max_error_number" = "1000",
"max_filter_ratio" = "0.1",
"timezone" = "UTC",
"partial_columns" = "true"
)
FROM KAFKA
(
"kafka_broker_list" = "172.30.180.22:29092",
"kafka_topic" = "test_user_topic",
"property.kafka_default_offsets" = "OFFSET_END",
"property.group.id" = "routine-load-consumer"
);

after sending partial update json string {"id":1,"age":25} to kafka and other columns in existing row are updated to NULL.

### What You Expected?

other columns keep original value.

### How to Reproduce?

_No response_

### Anything Else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)