Add support for null values in `Datum`

In #1824 we use instances of `Datum` to represent constants which can be added either by partition data or by user request as columns in the record batch transformer. However, `Datum` does not support null values. See [this](https://github.com/apache/iceberg-rust/pull/1824#discussion_r2584497197) discussion thread.

Start by reading the Datum implementation and the record batch transformer context described in issue #1824, then review the linked discussion about null values. Define the required null representation and verify that constants from partition data or user requests can be handled as null values in transformed record batches.

Written by the indexing model from the issue text.