fix(manifest): write null instead of empty list/map for unset optional DataFile fields

[Angelia-Wang]

What

AppendIntList, AppendIntMap, and AppendBinaryMap in src/iceberg/arrow_row_builder.cc always call ArrowArrayFinishElement() after appending entries, even when the input container is empty. As a result, whenever one of DataFile's optional list/map fields is unset, the C++ writer encodes an empty-but-non-null list/map element in the Avro manifest instead of a null element.

These three helpers back all of DataFile's optional list/map fields in manifest_adapter.cc:

• split_offsets, equality_ids (optional list)
• column_sizes, value_counts, null_value_counts, nan_value_counts, lower_bounds, upper_bounds (optional map)

Why

The Java reference implementation treats "unset" as null for these same fields: BaseFile initializes columnSizes, valueCounts, nullValueCounts, nanValueCounts, lowerBounds, upperBounds, splitOffsets, and equalityIds to null (not empty containers), and its get(pos, ...) accessor — used by the Avro manifest writer — returns null for an unset field (https://github.com/apache/iceberg/blob/1.1.x/core/src/main/java/org/apache/iceberg/BaseFile.java).
So manifests written by iceberg-cpp currently diverge from manifests written by iceberg-java whenever any of these 8 fields is unset.

This divergence is not cosmetic — it causes silent data loss when the table is read back with iceberg-java. We verified this end-to-end: after iceberg-cpp commits a snapshot, reading the table with IcebergGenerics.read(table) (iceberg 1.1.0) returns 0 rows instead of the committed rows:

1. split_offsets unset → encoded as empty Avro array [] instead of null.
2. BaseFile.splitOffsets() converts the empty array into a non-null empty List<Long> (https://github.com/apache/iceberg/blob/1.1.x/core/src/main/java/org/apache/iceberg/BaseFile.java).
3. BaseContentScanTask.split() selects OffsetsAwareSplitScanTaskIterator, because file.splitOffsets() != null && OFFSET_ORDERING.isOrdered(...) holds for an empty list (https://github.com/apache/iceberg/blob/1.1.x/core/src/main/java/org/apache/iceberg/BaseContentScanTask.java).
4. OffsetsAwareSplitScanTaskIterator produces zero split tasks for an empty offset list: splitSizes stays empty and hasNext() always returns false (https://github.com/apache/iceberg/blob/1.1.x/core/src/main/java/org/apache/iceberg/OffsetsAwareSplitScanTaskIterator.java).
5. The scan silently reads nothing — no exception, no warning.

With this change (null instead of empty list), the same end-to-end test reads back the expected rows: a null split_offsets makes BaseContentScanTask.split() fall back to FixedSizeSplitScanTaskIterator and the file is read normally.

How

Add an empty check to each of the three helpers that delegates to the existing AppendNull() instead of finishing an empty element. This covers all 8 optional list/map fields with one consistent change; manifest_adapter.cc is untouched. AppendStringMap (used for required properties-style maps) is intentionally left unchanged: an empty-but-non-null map is the expected encoding for required maps (covered by the existing ArrowRowBuilderTest.BuildsRowsWithTypedValues).

Testing

• New unit tests in arrow_row_builder_test.cc assert that AppendIntList/AppendIntMap/AppendBinaryMap write a null element (not an empty one) for empty input, alongside existing non-empty coverage.
• Existing manifest round-trip tests are unaffected: the C++ read path already normalizes both null and empty to empty containers.
• End-to-end verification (C++ writer → snapshot commit → Java IcebergGenerics.read reader): without the fix, 0 rows are read back; with the fix, all committed rows are read back.