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
if (fieldBlock.block().asVector() != null) {
                        return Block.Ref.floating(
                            BooleanBlock.newConstantBlockWith(true, page.getPositionCount(), driverContext.blockFactory())
                        );
                    }
                    try (
                        BooleanVector.FixedBuilder builder = BooleanVector.newVectorFixedBuilder(
                            page.getPositionCount(),
                            driverContext.blockFactory()
                        )
                    ) {
                        for (int p = 0; p < page.getPositionCount(); p++) {
                            builder.appendBoolean(fieldBlock.block().isNull(p) == false);
                        }
                        return Block.Ref.floating(builder.build().asBlock());
                    }
                }
            }
            @Override
            public void close() {
                Releasables.closeExpectNoException(field);
            }
            @Override
            public String toString() {
                return "IsNotNullEvaluator[field=" + field + ']';
            }
        }
    }
}