Describe the bug

UnionArray::is_null checks the Union's validity map rather than its slot.

Here is the default impl for is_null
https://github.com/apache/arrow-rs/blob/master/arrow/src/array/array.rs#L164-L166

There is no override in the union's impl of Array:https://github.com/apache/arrow-rs/blob/master/arrow/src/array/array_union.rs#L305-L313

See discussion from @tustvold and @viirya here: https://github.com/apache/arrow-rs/pull/1589/files#r854817015

To Reproduce
Steps to reproduce the behavior:

Expected behavior
UnionArray::is_null should check the slot (child's data) for nullness, rather than its own

Additional context
Add any other context about the problem here.