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
@Override
        public ExpressionEvaluator.Factory map(IsNull isNull, Layout layout) {
            var field = toEvaluator(isNull.field(), layout);
            return new IsNullEvaluatorFactory(field);
        }
        record IsNullEvaluatorFactory(EvalOperator.ExpressionEvaluator.Factory field) implements ExpressionEvaluator.Factory {
            @Override
            public ExpressionEvaluator get(DriverContext context) {
                return new IsNullEvaluator(context, field.get(context));
            }
            @Override
            public String toString() {
                return "IsNullEvaluator[field=" + field + ']';
            }
        }
        record IsNullEvaluator(DriverContext driverContext, EvalOperator.ExpressionEvaluator field) implements ExpressionEvaluator {
            @Override
            public Block.Ref eval(Page page) {