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

### 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`:

```java
StructVector parent = ...; // child "l" is LIST, valueCount == 0
parent.getReader().reader("l"); // IOOBE
```

Beginne mit UnionListReader.setPosition und verfolge die Aufrufe von SingleStructReaderImpl.reader(String); vergleiche die zugehörigen UnionMapReader- und UnionLargeListReader-Pfade, die im Issue beschrieben sind. Reproduziere den Fall einer leeren Liste nach IPC und überprüfe anschließend, dass diese Reader keine IndexOutOfBoundsException mehr auslösen, wenn Offsets oder die Kapazität leer oder kürzer als erwartet sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.