### Description

`from employees | eval x = 1 | keep x` results in 

```
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because the return value of "org.elasticsearch.xpack.esql.planner.Layout.getChannel(org.elasticsearch.xpack.ql.expression.NameId)" is null
        at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.alignPageToAttributes(LocalExecutionPlanner.java:281)
        at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.planExchangeSink(LocalExecutionPlanner.java:305)
        at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:199)
        at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:146)
        at org.elasticsearch.xpack.esql.plugin.ComputeService.runCompute(ComputeService.java:246)
        at org.elasticsearch.xpack.esql.plugin.ComputeService$DataNodeRequestHandler.lambda$messageReceived$5(ComputeService.java:415)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.action.ActionListener$2.onResponse(ActionListener.java:177)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.action.ActionListener.completeWith(ActionListener.java:305)
        at org.elasticsearch.xpack.esql.plugin.ComputeService.lambda$acquireSearchContexts$13(ComputeService.java:280)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.index.shard.IndexShard.ensureShardSearchActive(IndexShard.java:3911)
        at org.elasticsearch.xpack.esql.plugin.ComputeService.acquireSearchContexts(ComputeService.java:278)
        at org.elasticsearch.xpack.esql.plugin.ComputeService$DataNodeRequestHandler.messageReceived(ComputeService.java:413)
        at org.elasticsearch.xpack.esql.plugin.ComputeService$DataNodeRequestHandler.messageReceived(ComputeService.java:405)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:74)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.transport.TransportService$6.doRun(TransportService.java:1023)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:983)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.TimedRunnable.doRun(TimedRunnable.java:33)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:983)
        at org.elasticsearch.server@8.10.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
        at java.base/java.lang.Thread.run(Thread.java:1623)
```
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/CsvTests.java
private ActualResults executePlan() throws Exception {
        var parsed = parser.createStatement(testCase.query);
        var testDataset = testsDataset(parsed);
        String sessionId = "csv-test";
        ExchangeSourceHandler exchangeSource = new ExchangeSourceHandler(between(1, 64), threadPool.executor(ESQL_THREAD_POOL_NAME));
        ExchangeSinkHandler exchangeSink = new ExchangeSinkHandler(between(1, 64), threadPool::relativeTimeInMillis);
        LocalExecutionPlanner executionPlanner = new LocalExecutionPlanner(
            sessionId,
            new CancellableTask(1, "transport", "esql", null, TaskId.EMPTY_TASK_ID, Map.of()),
            BigArrays.NON_RECYCLING_INSTANCE,
            configuration,
            exchangeSource,
            exchangeSink,
            Mockito.mock(EnrichLookupService.class),
            testOperationProviders(testDataset)
        );
        //
        // 
        //
        var localTestOptimizer = new TestLocalPhysicalPlanOptimizer(new LocalPhysicalOptimizerContext(configuration));
        PhysicalPlan physicalPlan = physicalPlan(parsed, testDataset);
        Tuple<PhysicalPlan, PhysicalPlan> coordinatorAndDataNodePlan = CSVbreakPlanBetweenCoordinatorAndDataNode(
            physicalPlan,
            localTestOptimizer
        );
        PhysicalPlan coordinatorPlan = coordinatorAndDataNodePlan.v1();
        PhysicalPlan dataNodePlan = coordinatorAndDataNodePlan.v2();
        if (LOGGER.isTraceEnabled()) {
            LOGGER.trace("Coordinator plan\n" + coordinatorPlan);
            LOGGER.trace("DataNode plan\n" + dataNodePlan);