Core: Preserve null operations in inspect.snapshots

[fallintoplace]

Rationale for this change

The snapshots metadata table declares operation as nullable because snapshots without a summary do not have an operation. However, InspectTable.snapshots() converted the missing value with str(None), returning the literal string "None" instead of null.

This change passes the operation directly to PyArrow, preserving null semantics and matching Apache Iceberg Java's snapshots metadata table.

Are these changes tested?

Yes. A regression test creates a snapshot without a summary and verifies that table.inspect.snapshots() returns a null operation.

The following checks pass locally:

• PYTHONPATH=. uv run pytest tests/table/test_inspect.py -q (5 passed)
• PYTHONPATH=. uv run --extra datafusion --extra pyiceberg-core pytest tests/table -q (328 passed)
• make lint

Are there any user-facing changes?

Yes. For snapshots without a summary, table.inspect.snapshots() now returns null in the operation column instead of the string "None". There are no API changes.

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that's incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.

[Fokko]

Reasonable change, thanks @fallintoplace

[rambleraptor]

@fallintoplace it looks like this is ready to merge. Can you fix the linters so we can get a successful CI run?

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that's incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.