arrow

GH-31621: [JS] Fix Union null bitmaps (#37122)

This PR fixes `Union` null handling, and re-enables the disabled `SparseUnion` test.

The format doc [says](https://arrow.apache.org/docs/format/Columnar.html#union-layout):
> Unlike other data types, unions do not have their own validity bitmap.

Therefore we need to remove null masks from union types and allow them to delegate validity to their children.

Also fixes https://github.com/apache/arrow/issues/37063 for good measure.

* Closes: #31621
* Closes: #37063
* Closes #24123
* Closes #17168

Authored-by: ptaylor <paul.e.taylor@me.com>
Signed-off-by: Dominik Moritz <domoritz@gmail.com>