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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */