`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
```

从 UnionListReader.setPosition 开始，跟踪从 SingleStructReaderImpl.reader(String) 发出的调用；对比 issue 中描述的相关 UnionMapReader 和 UnionLargeListReader 路径。复现 IPC 后列表为空的情况，然后验证当 offsets 或 capacity 为空或短于预期时，这些 reader 不再抛出 IndexOutOfBoundsException。

由索引模型根据 Issue 内容生成。