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(Literal lit, Layout layout) {
            record LiteralsEvaluator(DriverContext context, Literal lit) implements ExpressionEvaluator {
                @Override
                public Block.Ref eval(Page page) {
                    return Block.Ref.floating(block(lit, context.blockFactory(), page.getPositionCount()));
                }
                @Override
                public String toString() {
                    return "LiteralsEvaluator[lit=" + lit + ']';
                }
                @Override
                public void close() {}