get NullPointerException when org.apache.shardingsphere.sharding.merge.dql.groupby.GroupByMemoryMergedResult.getValueCaseSensitiveFromTables(GroupByMemoryMergedResult.java:135)

Nobody has claimed this yet.

## Bug Report

### Which version of ShardingSphere did you use?
5.1.1 and 5.2.1

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

### Expected behavior
query success

### Actual behavior
java.lang.NullPointerException: null
at org.apache.shardingsphere.sharding.merge.dql.groupby.GroupByMemoryMergedResult.getValueCaseSensitiveFromTables(GroupByMemoryMergedResult.java:135) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.groupby.GroupByMemoryMergedResult.getValueCaseSensitive(GroupByMemoryMergedResult.java:125) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.groupby.GroupByMemoryMergedResult.init(GroupByMemoryMergedResult.java:73) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.groupby.GroupByMemoryMergedResult.init(GroupByMemoryMergedResult.java:53) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.infra.merge.result.impl.memory.MemoryMergedResult.(MemoryMergedResult.java:52) ~[shardingsphere-infra-merge-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.groupby.GroupByMemoryMergedResult.(GroupByMemoryMergedResult.java:56) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.ShardingDQLResultMerger.getGroupByMergedResult(ShardingDQLResultMerger.java:118) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.ShardingDQLResultMerger.build(ShardingDQLResultMerger.java:88) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.sharding.merge.dql.ShardingDQLResultMerger.merge(ShardingDQLResultMerger.java:66) ~[shardingsphere-sharding-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.infra.merge.MergeEngine.executeMerge(MergeEngine.java:82) ~[shardingsphere-infra-merge-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.infra.merge.MergeEngine.merge(MergeEngine.java:71) ~[shardingsphere-infra-merge-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.driver.jdbc.core.statement.ShardingSpherePreparedStatement.mergeQuery(ShardingSpherePreparedStatement.java:517) ~[shardingsphere-jdbc-core-5.2.1.jar:5.2.1]
at org.apache.shardingsphere.driver.jdbc.core.statement.ShardingSpherePreparedStatement.getResultSet(ShardingSpherePreparedStatement.java:465) ~[shardingsphere-jdbc-core-5.2.1.jar:5.2.1]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_272]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_272]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_272]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_272]
at org.apache.ibatis.logging.jdbc.PreparedStatementLogger.invoke(PreparedStatementLogger.java:69) ~[mybatis-3.5.4.jar:3.5.4]
at com.sun.proxy.$Proxy90.getResultSet(Unknown Source) ~[?:?]
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getFirstResultSet(DefaultResultSetHandler.java:237) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSets(DefaultResultSetHandler.java:187) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:65) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:79) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:63) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:76) ~[mybatis-3.5.4.jar:3.5.4]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_272]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_272]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_272]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_272]
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:426) ~[mybatis-spring-2.0.4.jar:2.0.4]
at com.sun.proxy.$Proxy80.selectOne(Unknown Source) ~[?:?]
at org.mybatis.spring.SqlSessionTemplate.selectOne(SqlSessionTemplate.java:159) ~[mybatis-spring-2.0.4.jar:2.0.4]
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:87) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.binding.MapperProxy$PlainMethodInvoker.invoke(MapperProxy.java:144) ~[mybatis-3.5.4.jar:3.5.4]
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:85) ~[mybatis-3.5.4.jar:3.5.4]
at com.sun.proxy.$Proxy82.selectCount(Unknown Source) ~[?:?]
at com.shardingtest1.control.User1Control.count(User1Control.java:41) ~[classes/:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_272]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_272]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_272]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_272]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-5.3.14.jar:5.3.14]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150) ~[spring-web-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.14.jar:5.3.14]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.3.14.jar:5.3.14]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:655) ~[tomcat-embed-core-9.0.56.jar:4.0.FR]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.3.14.jar:5.3.14]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:764) ~[tomcat-embed-core-9.0.56.jar:4.0.FR]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.56.jar:9.0.56]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.14.jar:5.3.14]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.14.jar:5.3.14]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.14.jar:5.3.14]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.14.jar:5.3.14]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.14.jar:5.3.14]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.14.jar:5.3.14]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:540) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:895) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1732) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) [tomcat-embed-core-9.0.56.jar:9.0.56]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.56.jar:9.0.56]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_272]

### Reason analyze (If you can)
My Database is PG，get defaule schema is public，

The result of selectStatementContext.getTablesContext().getSchemaName() is null

So get schema from database us default schema of public

The result of schema is empty

when merge result, cannot get table info from schema

cause null exception

### Steps to reproduce the behavior, such as: SQL to execute, sharding rule configuration, when exception occur etc.
spring:
shardingsphere:
enabled: true
schema:
name: mescenter_tddm
datasource:
names: ds0,ds1
ds0:
type: org.apache.commons.dbcp2.BasicDataSource
driver-class-name: org.postgresql.Driver
url: jdbc:postgresql://10.10.10.10:5432/tgmescenter_dlv00?targetServerType=master
username: ******
password: ******
ds1:
type: org.apache.commons.dbcp2.BasicDataSource
driver-class-name: org.postgresql.Driver
url: jdbc:postgresql://10.10.10.11:5432/tgmescenter_dlv01?targetServerType=master
username: ******
password: ******
props:
sql-show: true
sharding-route: true
rules:
sharding:
tables:
data_test:
actual-data-nodes: ds$->{0..1}.data_test
database-strategy:
standard:
sharding-column: userid
sharding-algorithm-name: sn_data_algorithm
sharding-algorithms:
sn_data_algorithm:
type: INLINE
props:
algorithm-expression: ds$->{userid % 2}

SQL：
\\
select count(1) as cnt from data_test a
\\
### Example codes for reproduce this issue (such as a github link).

Read GroupByMemoryMergedResult.java around line 135 and trace the GROUP BY merge path shown in the stack trace. The report lacks a reproducible SQL query or test location, so first reproduce the ShardingSphere-JDBC failure for the reported versions. Done means the query completes successfully without the NullPointerException, with a regression test for the case.

Written by the indexing model from the issue text.