Describe the bug

Unlike other data types, unions do not have their own validity bitmap. Instead, the nullness of each slot is determined exclusively by the child arrays which are composed to create the union.

From https://arrow.apache.org/docs/format/Columnar.html#union-layout

To Reproduce

We currently support this, which aside from adding a lot of complexity, is incorrect

Expected behavior

We shouldn't support something not in the arrow specification