DistSQL pymysql execute error

## Bug Report

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### Which version of ShardingSphere did you use?

5.5.0
pymysql 0.10.1

### Which project did you use? ShardingSphere-JDBC or ShardingSphere-Proxy?

shardingsphere-proxy

1. pymysql connect shardingsphere-proxy ----- success

2. pymysql create database and use database. pymysql.connect.commit() ----- success

3. pymysql register two storage unit ----- success

4. show storage units ----- success

5. unregister first storage unit ------ failed
pass first exception. and continue unregister second storage unit --- success

### Expected behavior

unregister success

### Actual behavior

unregister failed .

### Reason analyze (If you can)

[ERROR] 2024-05-17 14:45:33.802 [ShardingSphere-Command-16] o.a.s.p.f.c.CommandExecutorTask - Exception occur:
java.lang.NullPointerException: null
at org.apache.shardingsphere.infra.datasource.pool.CatalogSwitchableDataSource.getConnection(CatalogSwitchableDataSource.java:46)
at org.apache.shardingsphere.infra.database.DatabaseTypeEngine.getStorageType(DatabaseTypeEngine.java:118)
at org.apache.shardingsphere.infra.database.DatabaseTypeEngine.getProtocolType(DatabaseTypeEngine.java:62)
at org.apache.shardingsphere.metadata.factory.ExternalMetaDataFactory.create(ExternalMetaDataFactory.java:54)
at org.apache.shardingsphere.mode.manager.context.ConfigurationContextManager.createChangedDatabase(ConfigurationContextManager.java:334)
at org.apache.shardingsphere.mode.manager.context.ConfigurationContextManager.createChangedDatabases(ConfigurationContextManager.java:306)
at org.apache.shardingsphere.mode.manager.context.ConfigurationContextManager.createMetaDataContexts(ConfigurationContextManager.java:282)
at org.apache.shardingsphere.mode.manager.standalone.StandaloneModeContextManager.unregisterStorageUnits(StandaloneModeContextManager.java:246)
at org.apache.shardingsphere.distsql.handler.executor.rdl.resource.UnregisterStorageUnitExecutor.executeUpdate(UnregisterStorageUnitExecutor.java:60)
at org.apache.shardingsphere.distsql.handler.executor.rdl.resource.UnregisterStorageUnitExecutor.executeUpdate(UnregisterStorageUnitExecutor.java:47)
at org.apache.shardingsphere.distsql.handler.engine.update.DistSQLUpdateExecuteEngine.executeNormalUpdate(DistSQLUpdateExecuteEngine.java:86)
at org.apache.shardingsphere.distsql.handler.engine.update.DistSQLUpdateExecuteEngine.executeUpdate(DistSQLUpdateExecuteEngine.java:64)
at org.apache.shardingsphere.proxy.backend.handler.distsql.DistSQLUpdateBackendHandler.execute(DistSQLUpdateBackendHandler.java:45)
at org.apache.shardingsphere.proxy.frontend.mysql.command.query.text.query.MySQLComQueryPacketExecutor.execute(MySQLComQueryPacketExecutor.java:86)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.doExecuteCommand(CommandExecutorTask.java:126)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.executeCommand(CommandExecutorTask.java:121)
at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.run(CommandExecutorTask.java:78)
at com.alibaba.ttl.TtlRunnable.run(TtlRunnable.java:60)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:829)

### Steps to reproduce the behavior, such as: SQL to execute, sharding rule configuration, when exception occur etc.

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

Start with the reported stack trace at CatalogSwitchableDataSource.getConnection and follow the unregister path through StandaloneModeContextManager.unregisterStorageUnits, ExternalMetaDataFactory.create, and DatabaseTypeEngine. Reproduce the two-storage-unit PyMySQL workflow against ShardingSphere-Proxy 5.5.0; done means the first storage-unit unregister completes without the reported NullPointerException and the second remains usable.

Written by the indexing model from the issue text.