[C++] drop_null kernel ignores nulls when values is an union or a REE array

### Describe the bug, including details regarding any error messages, version, and platform.

Unions and run-end-encoded don't have validity bitmaps (`buffers[0]`), so they are sometimes treated as "array without nulls". This happens in the `drop_null` function: so even when unions have nulls, the input -- `values` -- is returned unfiltered.

### Component(s)

C++