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

從 TestStructVector.testStructVectorTransferPairWithNullType 開始，並將其與 testStructVectorTransferPairWithExtensionType 進行比較。追蹤 StructVector.getTransferPair 到 StructVector.java 和 NullableStructWriter.java，然後執行針對性的 TestStructVector 測試；當包含 NullVector 的 struct 能夠在不出現 Unknown type: NULL 例外的情況下完成傳輸，且回歸測試通過時，即完成。

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

added test case to TestStructVector:

  @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.<init>(NullableStructWriter.java:279)
	at org.apache.arrow.vector.complex.StructVector.<init>(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