### 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/internalClusterTest/java/org/elasticsearch/xpack/esql/action/EsqlActionIT.java
int randomValue = randomIntBetween(0, 100);
            countValuesGreaterThanFifty += randomValue >= 50 ? 1 : 0;
            builder.startObject();
            {
                builder.field("data", randomValue);
                builder.startArray("nested");
                {
                    for (int k = 0, max = randomIntBetween(1, 5); k < max; k++) {
                        
                        builder.startObject().field("foo", randomIntBetween(1000, 10000)).endObject();
                    }
                }
                builder.endArray();
            }
            builder.endObject();
            bulkBuilder.add(new IndexRequest(indexName).id(Integer.toString(j)).source(builder));
        }
        bulkBuilder.setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE).get();
        ensureYellow(indexName);
        return countValuesGreaterThanFifty;
    }
    private void createAlias(List<String> indices, String alias) throws InterruptedException, ExecutionException {
        IndicesAliasesRequest aliasesRequest = new IndicesAliasesRequest();
        for (String indexName : indices) {
            aliasesRequest.addAliasAction(IndicesAliasesRequest.AliasActions.add().index(indexName).alias(alias));
        }
        assertAcked(admin().indices().aliases(aliasesRequest).get());
    }
    private void assertNoNestedDocuments(String query, int docsCount, long minValue, long maxValue) {
        EsqlQueryResponse results = run(query);