`UnionListReader.setPosition` throws IOOBE on a post-IPC empty List

[bodduv]

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

On 19.0.0 (and on master), calling setPosition on a ListVector that came off the wire via IPC with valueCount == 0 throws IndexOutOfBoundsException.

Some callers may hit this without touching setPosition themselves as

• SingleStructReaderImpl.reader(String) calls setPosition(idx()) the first time a child reader on a struct is looked up.
• UnionMapReader inherits from UnionListReader and breaks the same way.
• UnionLargeListReader has a related variant; capacity guard seems to be missing and it fails whenever the offset buffer is shorter than (idx + 2) * 8 bytes.

The reader code hasn't changed. ListVector.setReaderAndWriterIndex changes and now correctly emits the leading [0] offset when valueCount == 0 (as per the Arrow spec), so the receiver's offset buffer is no longer "zero-capacity" and if block capacity() == 0 never enters.

Repro

The shape we actually hit — a struct with a list-typed child, struct itself having valueCount == 0:

StructVector parent = ...;            // child "l" is LIST<INT>, valueCount == 0
parent.getReader().reader("l");       // IOOBE