Fix SHOW MIGRATION RULE always display default values when ALTER MIGRATION RULE without STREAM_CHANNEL

[sandynz]

Related to #32727

Changes proposed in this pull request:

• Fix SHOW MIGRATION RULE always display default values when ALTER MIGRATION RULE without STREAM_CHANNEL

---

Issue:

mysql> SHOW MIGRATION RULE;
+--------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
| read                                                         | write                                | stream_channel                                        |
+--------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
| {"workerThread":20,"batchSize":1000,"shardingSize":10000000} | {"workerThread":20,"batchSize":1000} | {"type":"MEMORY","props":{"block-queue-size":"2000"}} |
+--------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
1 row in set (0.81 sec)

mysql> ALTER MIGRATION RULE (
    -> READ(
    ->   WORKER_THREAD=10,
    ->   BATCH_SIZE=2000,
    ->   SHARDING_SIZE=5000000,
    ->   RATE_LIMITER (TYPE(NAME='QPS',PROPERTIES('qps'='500')))
    -> )
    -> );
Query OK, 0 rows affected (0.10 sec)

-- It still show default values
mysql> SHOW MIGRATION RULE;
+--------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
| read                                                         | write                                | stream_channel                                        |
+--------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
| {"workerThread":20,"batchSize":1000,"shardingSize":10000000} | {"workerThread":20,"batchSize":1000} | {"type":"MEMORY","props":{"block-queue-size":"2000"}} |
+--------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
1 row in set (0.01 sec)

After fixing:

mysql> SHOW MIGRATION RULE;
+----------------------------------------------------------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
| read                                                                                                           | write                                | stream_channel                                        |
+----------------------------------------------------------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
| {"workerThread":10,"batchSize":2000,"shardingSize":5000000,"rateLimiter":{"type":"QPS","props":{"qps":"500"}}} | {"workerThread":20,"batchSize":1000} | {"type":"MEMORY","props":{"block-queue-size":"2000"}} |
+----------------------------------------------------------------------------------------------------------------+--------------------------------------+-------------------------------------------------------+
1 row in set (0.68 sec)

---

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