[arrow-vector] Zero-copy transfer of DecimalVector and Decimal256Vector is losing nullability information

[bodduv]

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

In the main branch, DecimalVector and Decimal256Vector are losing nullability information during a zero-copy transfer. It currently forces a nullable FieldType onto the "to" ValueVector.

The deficiency in the code is in DecimalVector.TransferImpl class when trying to get a transfer getTransferPair(String, BufferAllocator) creating a DecimalVector with a nullable FieldType (although the "from" ValueVector's FieldType is non-nullable):

arrow-java/vector/src/main/java/org/apache/arrow/vector/DecimalVector.java

Lines 563 to 569 in 48a20ba

	private class TransferImpl implements TransferPair {
	DecimalVector to;
	public TransferImpl(String ref, BufferAllocator allocator) {
	to =
	new DecimalVector(ref, allocator, DecimalVector.this.precision, DecimalVector.this.scale);
	}

Note that all the other (at least the primitive) ValueVectors are transferred without losing nullability information by using the "from" ValueVector's field. E.g.:

arrow-java/vector/src/main/java/org/apache/arrow/vector/BigIntVector.java

Lines 319 to 324 in 48a20ba

	private class TransferImpl implements TransferPair {
	BigIntVector to;
	public TransferImpl(String ref, BufferAllocator allocator) {
	to = new BigIntVector(ref, field.getFieldType(), allocator);
	}

---

To reproduce the issue: add the following test in src/test/java/org/apache/arrow/vector/TestDecimalVector.java to see it fail.

 @Test
 public void testGetTransferPairWithoutFieldNonNullable() {
 final FieldType decimalNonNullableType =
 new FieldType(false, new ArrowType.Decimal(10, scale), null);
 final DecimalVector fromVector =
 new DecimalVector("decimal", decimalNonNullableType, allocator);
 final TransferPair transferPair =
 fromVector.getTransferPair(fromVector.getField().getName(), allocator);
 final DecimalVector toVector = (DecimalVector) transferPair.getTo();

 // These asserts fail
 assertSame(fromVector.getField().isNullable(), toVector.getField().isNullable());
 assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
 }