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
}
            record LiteralsEvaluatorFactory(Literal lit) implements ExpressionEvaluator.Factory {
                @Override
                public ExpressionEvaluator get(DriverContext driverContext) {
                    return new LiteralsEvaluator(driverContext, lit);
                }
                @Override
                public String toString() {
                    return "LiteralsEvaluator[lit=" + lit + "]";
                }
            }
            return new LiteralsEvaluatorFactory(lit);
        }
        private static Block block(Literal lit, BlockFactory blockFactory, int positions) {
            var value = lit.value();
            if (value == null) {
                return Block.constantNullBlock(positions, blockFactory);
            }
            if (value instanceof List<?> multiValue) {
                if (multiValue.isEmpty()) {
                    return Block.constantNullBlock(positions, blockFactory);
                }
                var wrapper = BlockUtils.wrapperFor(blockFactory, ElementType.fromJava(multiValue.get(0).getClass()), positions);
                wrapper.accept(multiValue);
                return wrapper.builder().build();
            }
            return BlockUtils.constantBlock(blockFactory, value, positions);
        }
    }
    static class IsNulls extends ExpressionMapper<IsNull> {