GH-1125: Read empty ListVector correctly using UnionListReader after IPC deser

[bodduv]

Arrow IPC represents variable-length vectors with an offset buffer containing valueCount + 1 offsets. For an empty ListVector, that still means the serialized and deserialized vector can have a non-empty offset buffer containing the leading zero offset. This is correct according to the Arrow layout, but it exposes a bug at UnionListReader.setPosition and other similar places. UnionListReader.setPosition(0) used offset-buffer capacity as the empty-vector check. That worked only when the offset buffer had zero capacity. After IPC, the empty vector has non-zero offset-buffer capacity, so the reader could throw IndexOutOfBoundsException. UnionLargeListReader has the same logical issue and also lacked the empty-buffer guard.

What's Changed

Update UnionListReader and UnionLargeListReader to validate reader positioning against valueCount instead of treating offset-buffer capacity as the logical row boundary. All out-of-range positions will throw. For valid non-empty positions, the readers also defensively verify that the offset buffer has enough capacity for both index and index + 1 before reading offsets.

The shared bounds logic is kept in a package-private UnionListReaderBoundsChecker helper so UnionListReader and UnionLargeListReader use the reuse code.

Closes #1125.

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[bodduv]

I don't think this is a breaking-change, it should be labeled bug-fix