### Elasticsearch Version

main

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

all

### Problem Description

Do this

### Steps to Reproduce

```
rm /tmp/evil
for a in {0..9}; do
   for b in {0..9}; do
      echo '{"index": {}}' >> /tmp/evil
      echo '{"a": '$a', "b": '$b'}' >> /tmp/evil
   done
done
echo >> /tmp/evil

curl -uelastic:password -XDELETE localhost:9200/test
curl -HContent-Type:application/json -uelastic:password -XPUT localhost:9200/test
curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/test/_bulk --data-binary @/tmp/evil
curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/test/_forcemerge?max_num_segments=1
curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/test/_refresh

curl -HContent-Type:application/json -uelastic:password -XPOST localhost:9200/_esql?pretty -d'{
   "query": "FROM test | STATS MAX(a) BY a, b | KEEP a"
}'
```

### Logs (if relevant)

_No response_
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/ProjectOperator.java
private final BitSet bs;
    private Block[] blocks;
    public record ProjectOperatorFactory(BitSet mask) implements OperatorFactory {
        @Override
        public Operator get(DriverContext driverContext) {
            return new ProjectOperator(mask);
        }
        @Override
        public String describe() {
            return "ProjectOperator[mask = " + mask + "]";
        }
    }
    /**
     * Creates a project that applies the given mask (as a bitset).
     *
     * @param mask bitset mask for enabling/disabling blocks / columns inside a Page
     */
    public ProjectOperator(BitSet mask) {
        this.bs = mask;
    }
    @Override
    protected Page process(Page page) {
        if (page.getBlockCount() == 0) {
            return page;
        }