[arrow] Use the nested field's own nullability for sub writers

[thswlsqls]

Purpose

fix #8757

• ArrowFieldWriterFactoryVisitor passed the parent field's isNullable to ARRAY/VECTOR/MAP/ROW sub writers, and ArrowFieldWriter.write rejects null with IllegalArgumentException when that flag is false. An ARRAY<INT> NOT NULL column therefore rejected valid null elements, while a NOT NULL child under a nullable parent silently accepted nulls.
• Each sub writer now uses its own type's isNullable(), matching the child nullability ArrowUtils.toArrowField() declares.
• Same rule applied to ArrowUtils.createArrowFieldWriters() (test-only caller).
• Map keys follow the key type; toArrowField() declares the Arrow key field not-null, left as a follow-up.
• Follow-up of [arrow] Fix bug: field writer contruct in ArrowFormatWriter should use nested field nullable to create sub writers #5538, which fixed only ArrowFormatWriter.

Tests

• Added four ArrowFormatWriterTest cases covering null children under NOT NULL ARRAY/MAP columns and NOT NULL ARRAY elements / nested ROW fields. All four fail before the change.
• mvn -pl paimon-arrow -am clean install — 52 tests passed.
• mvn -pl paimon-core -Dtest=ArrowBatchConverterTest test — 21 tests passed.

[JingsongLi]

• ArrowFieldWriterFactoryVisitor.visit(MapType) changes the key writer’s nullability to mapType.getKeyType().isNullable(), but ArrowUtils.toArrowField() always declares the Arrow map key schema as NOT NULL.
• Paimon’s MapType explicitly allows null keys; the default INT key is also nullable. Consequently, MAP<INT, INT> NOT NULL can now successfully write null keys, generating data that conflicts with the Arrow schema/spec.
• The local null-key use case has been reproduced: it is expected to throw an exception, but no exception is actually thrown. It is recommended that the key writer consistently use false; null keys that cannot be expressed should result in a “fail loud” error.

[thswlsqls]

Thanks, good catch. You're right — ArrowUtils.toArrowField() forces the map key to notNull(), so the key writer has to stay non-nullable regardless of keyType.isNullable(); otherwise a null key gets written instead of failing loud, which contradicts the schema. Fixed — the key writer is back to non-nullable (the value, array-element, and row-field branches still mirror each nested type's own nullability, which is what the schema declares for them), and I added a null-key test. Just pushed.

[JingsongLi]

+1