Null fields from origin to unset at target cluster does not work in all cases

Hi, Since 5.4.0 CDM should be able to `unset()` null field to avoid creating tombstones during migration: https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/statements/prepared/index.html#unset-values It seems that the feature has been partially reverted in later "unrelated" commit/versions (no more `unset()` usage): https://github.com/datastax/cassandra-data-migrator/commit/7894842021a87eac0154a32bd977bb70a40d494f Also, migrating collections (i.e `id_lists set<ascii>`) creates tombstones when content is `null`, by writing `id_lists: {}`, instead of using `unset()`. I have a local modification to CDM to check for empty collection, and call the `unset()` feature on `boundStatement` (I have billions of rows to migrate with empty collections so this is really useful for us) Before opening a PR, I want to understand if the current behavior/revert is desired on your side, or if I'm misunderstand something.