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(Attribute attr, Layout layout) {
            record Attribute(int channel) implements ExpressionEvaluator {
                @Override
                public Block.Ref eval(Page page) {
                    return new Block.Ref(page.getBlock(channel), page);
                }
                @Override
                public void close() {}
            }
            record AttributeFactory(int channel) implements ExpressionEvaluator.Factory {
                @Override
                public ExpressionEvaluator get(DriverContext driverContext) {
                    return new Attribute(channel);
                }
                @Override
                public String toString() {
                    return "Attribute[channel=" + channel + "]";
                }
            }
            return new AttributeFactory(layout.get(attr.id()).channel());
        }
    }
    static class Literals extends ExpressionMapper<Literal> {