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
}
                }
                private Block eval(BooleanVector lhs, BooleanVector rhs) {
                    int positionCount = lhs.getPositionCount();
                    try (var result = BooleanVector.newVectorFixedBuilder(positionCount, lhs.blockFactory())) {
                        for (int p = 0; p < positionCount; p++) {
                            result.appendBoolean(bl.function().apply(lhs.getBoolean(p), rhs.getBoolean(p)));
                        }
                        return result.build().asBlock();
                    }
                }
                @Override
                public void close() {
                    Releasables.closeExpectNoException(leftEval, rightEval);
                }
            }
            return driverContext -> new BooleanLogicExpressionEvaluator(bc, leftEval.get(driverContext), rightEval.get(driverContext));
        }
    }
    static class Nots extends ExpressionMapper<Not> {
        @Override
        public ExpressionEvaluator.Factory map(Not not, Layout layout) {
            var expEval = toEvaluator(not.field(), layout);
            return dvrCtx -> new org.elasticsearch.xpack.esql.evaluator.predicate.operator.logical.NotEvaluator(
                expEval.get(dvrCtx),
                dvrCtx
            );
        }
    }
    static class Attributes extends ExpressionMapper<Attribute> {