A query like 
```
from employees | eval foldable = \"foo,bar\" | eval x = split(foldable, \",\")
```

where the `split()` function is applied to a foldable attribute, throws IllegalArgumentException `Block does not have same position count`

The same expression applied to non-foldable attributes seems to work fine eg.

```
from employees | project first_name |  eval x = split(first_name, \"e\")
```

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/evaluator/EvalMapper.java
package org.elasticsearch.xpack.esql.evaluator;
import org.elasticsearch.compute.ann.Evaluator;
import org.elasticsearch.compute.data.Block;
import org.elasticsearch.compute.data.BlockFactory;
import org.elasticsearch.compute.data.BlockUtils;
import org.elasticsearch.compute.data.BooleanBlock;
import org.elasticsearch.compute.data.BooleanVector;
import org.elasticsearch.compute.data.ElementType;
import org.elasticsearch.compute.data.Page;
import org.elasticsearch.compute.data.Vector;
import org.elasticsearch.compute.operator.DriverContext;
import org.elasticsearch.compute.operator.EvalOperator;
import org.elasticsearch.compute.operator.EvalOperator.ExpressionEvaluator;
import org.elasticsearch.core.Releasables;
import org.elasticsearch.xpack.esql.evaluator.mapper.EvaluatorMapper;
import org.elasticsearch.xpack.esql.evaluator.mapper.ExpressionMapper;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.ComparisonMapper;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.InMapper;
import org.elasticsearch.xpack.esql.evaluator.predicate.operator.regex.RegexMapper;
import org.elasticsearch.xpack.esql.planner.Layout;
import org.elasticsearch.xpack.ql.QlIllegalArgumentException;
import org.elasticsearch.xpack.ql.expression.Attribute;
import org.elasticsearch.xpack.ql.expression.Expression;
import org.elasticsearch.xpack.ql.expression.Literal;
import org.elasticsearch.xpack.ql.expression.predicate.logical.BinaryLogic;
import org.elasticsearch.xpack.ql.expression.predicate.logical.Not;
import org.elasticsearch.xpack.ql.expression.predicate.nulls.IsNotNull;
import org.elasticsearch.xpack.ql.expression.predicate.nulls.IsNull;
import java.util.List;
public final class EvalMapper {