'Unknown type: NULL' when initialising a NullableStructWriter from a struct with NullVector children

### Describe the bug, including details regarding any error messages, version, and platform.

added test case to TestStructVector:

```java

@Test
public void testStructVectorTransferPairWithNullType() {
Field uuidField = new Field("null", FieldType.nullable(ArrowType.Null.INSTANCE), null);
Field structField =
new Field("struct", FieldType.nullable(new ArrowType.Struct()), List.of(uuidField));

try(StructVector s1 = (StructVector) structField.createVector(allocator)) {
NullVector nullVector =
s1.addOrGet("null", FieldType.nullable(ArrowType.Null.INSTANCE), NullVector.class);
s1.setValueCount(1);
nullVector.setNull(0);
s1.setIndexDefined(0);

TransferPair tp = s1.getTransferPair(structField, allocator);
try (StructVector toVector = (StructVector) tp.getTo()) {
assertEquals(s1.getField(), toVector.getField());
}
}
}
```

got this:

```
java.lang.UnsupportedOperationException: Unknown type: NULL
at org.apache.arrow.vector.complex.impl.NullableStructWriter.(NullableStructWriter.java:279)
at org.apache.arrow.vector.complex.StructVector.(StructVector.java:74)
at org.apache.arrow.vector.complex.StructVector.getTransferPair(StructVector.java:261)
at org.apache.arrow.vector.TestStructVector.testStructVectorTransferPairWithNullType(TestStructVector.java:389)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
```

expected: similar result to `testStructVectorTransferPairWithExtensionType`

Cheers!

James

our issue: xtdb/xtdb#4693

Inizia con TestStructVector.testStructVectorTransferPairWithNullType e confrontalo con testStructVectorTransferPairWithExtensionType. Segui StructVector.getTransferPair in StructVector.java e NullableStructWriter.java, quindi esegui i test mirati di TestStructVector; completato quando una struct contenente un NullVector viene trasferita senza l'eccezione Unknown type: NULL e il test di regressione ha esito positivo.

Scritto dal modello di indicizzazione a partire dal testo della issue.