java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null

## Bug Report

### Which version of ShardingSphere did you use?
f306248c8d1d7e4b54ffec60eb4a125eb0f52119
### Which project did you use? ShardingSphere-JDBC or ShardingSphere-Proxy?
ShardingSphere-Proxy
### Expected behavior
have no error
### Actual behavior
```
[ERROR] 2022-12-21 18:42:07.195 [ShardingSphere-Command-183] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.binary.execute.MySQLComStmtExecuteExecutor.close(MySQLComStmtExecuteExecutor.java:130)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.executeCommand(CommandExecutorTask.java:124)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.run(CommandExecutorTask.java:78)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
[ERROR] 2022-12-21 18:42:07.195 [ShardingSphere-Command-75] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.binary.execute.MySQLComStmtExecuteExecutor.close(MySQLComStmtExecuteExecutor.java:130)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.executeCommand(CommandExecutorTask.java:124)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.run(CommandExecutorTask.java:78)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
[ERROR] 2022-12-21 18:42:07.195 [ShardingSphere-Command-164] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.binary.execute.MySQLComStmtExecuteExecutor.close(MySQLComStmtExecuteExecutor.java:130)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.executeCommand(CommandExecutorTask.java:124)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.run(CommandExecutorTask.java:78)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
[ERROR] 2022-12-21 18:42:07.195 [ShardingSphere-Command-157] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.binary.execute.MySQLComStmtExecuteExecutor.close(MySQLComStmtExecuteExecutor.java:130)

[ERROR] 2022-12-21 18:42:07.195 [ShardingSphere-Command-164] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.binary.execute.MySQLComStmtExecuteExecutor.close(MySQLComStmtExecuteExecutor.java:130)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.executeCommand(CommandExecutorTask.java:124)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.run(CommandExecutorTask.java:78)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
[ERROR] 2022-12-21 18:42:07.195 [ShardingSphere-Command-157] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: Cannot invoke "org.apache.shardingsphere.proxy.backend.handler.ProxyBackendHandler.close()" because "this.proxyBackendHandler" is null
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.binary.execute.MySQLComStmtExecuteExecutor.close(MySQLComStmtExecuteExecutor.java:130)

...
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-192] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-161] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-155] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-75] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-75] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-192] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-75] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.199 [ShardingSphere-Command-147] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.200 [ShardingSphere-Command-189] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.200 [ShardingSphere-Command-154] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.202 [ShardingSphere-Command-189] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.202 [ShardingSphere-Command-163] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.202 [ShardingSphere-Command-155] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.202 [ShardingSphere-Command-196] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.202 [ShardingSphere-Command-161] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.203 [ShardingSphere-Command-151] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.203 [ShardingSphere-Command-196] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.203 [ShardingSphere-Command-189] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.203 [ShardingSphere-Command-195] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
[ERROR] 2022-12-21 18:42:07.203 [ShardingSphere-Command-132] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
```

### Reason analyze (If you can)
can't
### Steps to reproduce the behavior, such as: SQL to execute, sharding rule configuration, when exception occur etc.
1. start proxy
2. config proxy
```
databaseName: sharding_db
dataSources:
ds_0:
password:
url: jdbc:mysql://192.168.10.23:13333/test?serverTimezone=UTC&useSSL=false
username: root
minPoolSize: 1
connectionTimeoutMilliseconds: 30000
maxLifetimeMilliseconds: 2100000
readOnly: false
idleTimeoutMilliseconds: 30000
maxPoolSize: 50
rules:
```
3. use sysbench threads=40
```
sysbench oltp_read_write --mysql-host=192.168.10.23 --mysql-port=6666 --mysql-user=root --mysql-password=root --mysql-db=sharding_db --tables=1 --table-size=1000 --report-interval=2 --time=1600 --threads=40 --max-requests=0 --mysql-ignore-errors=all --rand-type=uniform --range_selects=off --auto_inc=off run
```
4. proxy execute `show processlist;` count 40

5. proxy execute alter resource
```
ALTER STORAGE UNIT ds_0 (
URL="jdbc:mysql://192.168.10.23:13333/test?serverTimezone=UTC&useSSL=false",
USER="root",
PASSWORD="",
PROPERTIES("maximumPoolSize"="10","idleTimeout"="30000")
);
```
6. proxy execute `show processlist;` count 40

7. cat stdout.log

8. ALTER STORAGE UNIT ds_0 (
URL="jdbc:mysql://192.168.10.23:13333/test?serverTimezone=UTC&useSSL=false",
USER="root",
PASSWORD="",
PROPERTIES("maximumPoolSize"="50","idleTimeout"="30000")
);

9. cat stdout.log

sysbench resoult

### Example codes for reproduce this issue (such as a github link).

Start at MySQLComStmtExecuteExecutor.close(), line 130, and trace the cleanup path through CommandExecutorTask. Reproduce the failure with 40 sysbench threads while running ALTER STORAGE UNIT on ds_0, then verify that the sequence no longer emits the reported NullPointerException.

Written by the indexing model from the issue text.