NoSuchElementException when execute DALStatement without USE DB

[RaigorJiang]

Bug Report

Which version of ShardingSphere did you use?

5.5.2-SNAPSHOT a6a09ba

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

ShardingSphere-Proxy

Expected behavior

Query from unicast database

Actual behavior

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

1. Create logical database and register storage unit;
2. Quit current session;
3. Connect to Proxy and do not execute USE DB;
4. Execute SQL

show variables where Variable_name in ('explicit_defaults_for_timestamp', 'sql_mode');

5. Proxy log

java.util.NoSuchElementException: null
	at java.base/java.util.Collections$EmptyIterator.next(Collections.java:4310)
	at org.apache.shardingsphere.infra.session.query.QueryContext.getUsedDatabase(QueryContext.java:95)
	at org.apache.shardingsphere.proxy.backend.connector.StandardDatabaseConnector.<init>(StandardDatabaseConnector.java:126)
	at org.apache.shardingsphere.proxy.backend.connector.DatabaseConnectorFactory.newInstance(DatabaseConnectorFactory.java:52)
	at org.apache.shardingsphere.proxy.backend.handler.data.impl.UnicastDatabaseBackendHandler.execute(UnicastDatabaseBackendHandler.java:62)
	at org.apache.shardingsphere.proxy.frontend.mysql.command.query.text.query.MySQLComQueryPacketExecutor.execute(MySQLComQueryPacketExecutor.java:93)
	at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.doExecuteCommand(CommandExecutorTask.java:126)
	at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.executeCommand(CommandExecutorTask.java:121)
	at org.apache.shardingsphere.proxy.frontend.command.CommandExecutorTask.run(CommandExecutorTask.java:78)
	at com.alibaba.ttl.TtlRunnable.run(TtlRunnable.java:60)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:840)