### 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
var mapping = new TreeMap<>(loadMapping(mappingName));
        return IndexResolution.valid(new EsIndex(indexName, mapping, Set.of(indexName)));
    }
    private static EnrichResolution loadEnrichPolicies() {
        Set<String> names = new HashSet<>();
        Set<EnrichPolicyResolution> resolutions = new HashSet<>();
        for (CsvTestsDataLoader.EnrichConfig policyConfig : CsvTestsDataLoader.ENRICH_POLICIES) {
            EnrichPolicy policy = loadEnrichPolicyMapping(policyConfig.policyFileName());
            CsvTestsDataLoader.TestsDataset sourceIndex = CSV_DATASET_MAP.get(policy.getIndices().get(0));
            // 
            // 
            IndexResolution idxRes = loadIndexResolution(sourceIndex.mappingFileName(), sourceIndex.indexName());
            names.add(policyConfig.policyName());
            resolutions.add(new EnrichPolicyResolution(policyConfig.policyName(), policy, idxRes));
        }
        return new EnrichResolution(resolutions, names);
    }
    private static EnrichPolicy loadEnrichPolicyMapping(String policyFileName) {
        URL policyMapping = CsvTestsDataLoader.class.getResource("/" + policyFileName);
        assertThat(policyMapping, is(notNullValue()));
        try {
            String fileContent = CsvTestsDataLoader.readTextFile(policyMapping);
            return EnrichPolicy.fromXContent(JsonXContent.jsonXContent.createParser(XContentParserConfiguration.EMPTY, fileContent));
        } catch (IOException e) {
            throw new IllegalArgumentException("Cannot read resource " + policyFileName);
        }
    }
    private PhysicalPlan physicalPlan(LogicalPlan parsed, CsvTestsDataLoader.TestsDataset dataset) {
        var indexResolution = loadIndexResolution(dataset.mappingFileName(), dataset.indexName());
        var enrichPolicies = loadEnrichPolicies();