use autocommit txn, select system table CLUSTER_PROCESSLIST, txnStart does not show

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

session 1:
select * from gc_test_1650337277.t0 limit 1;

session 2:
select * from INFORMATION_SCHEMA.CLUSTER_PROCESSLIST where ID=1888875\G

### 2. What did you expect to see? (Required)

session 2:

mysql> select * from INFORMATION_SCHEMA.CLUSTER_PROCESSLIST where ID=1888875\G
*************************** 1. row ***************************
INSTANCE: 172.16.5.202:10180
ID: 1888875
USER: root
HOST: 172.16.4.202:46800
DB: INFORMATION_SCHEMA
COMMAND: Sleep
TIME: 3
STATE: autocommit
INFO: NULL
DIGEST: 810e7ba54a750f0cec84d37f9f850baba81585f8cb3db44aeb569c01f353545a
MEM: 0
DISK: 0
TxnStart: 04-19 04:06:20.235(432627038351524060)
1 row in set (0.01 sec)

### 3. What did you see instead (Required)

TxnStart is empty，as follow：
mysql> select * from INFORMATION_SCHEMA.CLUSTER_PROCESSLIST where ID=1888875\G
*************************** 1. row ***************************
INSTANCE: 172.16.5.202:10180
ID: 1888875
USER: root
HOST: 172.16.4.202:46800
DB: INFORMATION_SCHEMA
COMMAND: Sleep
TIME: 3
STATE: autocommit
INFO: NULL
DIGEST: 810e7ba54a750f0cec84d37f9f850baba81585f8cb3db44aeb569c01f353545a
MEM: 0
DISK: 0
TxnStart:
1 row in set (0.01 sec)

### 4. What is your TiDB version? (Required)

mysql> select tidb_version();
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v6.1.0-alpha
Edition: Community
Git Commit Hash: 32b9c14779c2a7dd73003667d81bb42f67a33385
Git Branch: heads/refs/tags/v6.1.0-alpha
UTC Build Time: 2022-04-11 15:00:38
GoVersion: go1.18
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)