[ump45nose]

What's Changed

NullableStructWriter / SingleStructWriter (generated from StructWriters.java) walk the struct field's children in their constructor and create a child vector for each one. There was no case NULL, so a struct whose field has a Null-typed child threw UnsupportedOperationException: Unknown type: NULL. Because StructVector builds its writer as soon as it is constructed, this broke new StructVector(field, ...) and anything that goes through it, such as StructVector#getTransferPair(Field, BufferAllocator) from the issue.

There is no writer for the Null type, so the new case only adds the NullVector child to the container (with the child's nullability). The resulting vector's field now matches the input field, and the other children's writers keep working as before.

Tests added to TestStructVector:

• testStructVectorTransferPairWithNullType: the reproducer from the issue, plus a transfer() that checks the value count, struct validity and the NullVector child.
• testStructVectorFromFieldWithNullChild: builds a StructVector straight from a Field with a Null child and an Int child, then writes through the struct writer.

Both fail on main with Unknown type: NULL and pass with this change. I also ran TestStructVector, TestComplexWriter, TestListVector, TestMapVector, TestUnionVector, TestDenseUnionVector, TestComplexCopier, TestPromotableWriter and TestSplitAndTransfer (all pass, both the default and netty runs), and mvn -pl vector spotless:check.

AI assistance disclosure: this change, its tests and this description were prepared with the help of an AI coding assistant. The tests listed above were run locally against this branch.

Closes #830.

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