Required fields nested within optional structs can produce nulls if the parent struct is null and fails `is_null` / `is_not_null` evaluation

### Apache Iceberg Rust version

v0.10.1

### Describe the bug

This [bug](https://github.com/apache/iceberg/issues/13328) is reported in Iceberg Java and and fixed in https://github.com/apache/iceberg/pull/14270

### To Reproduce

_No response_

### Expected behavior

_No response_

### Willingness to contribute

None

Start by comparing the Rust null-handling and is_null/is_not_null evaluation with Apache Iceberg Java issue 13328 and pull request 14270. Reproduce the case where a required field is nested in a null optional struct, then verify that evaluation no longer produces nulls.

Written by the indexing model from the issue text.