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

[Skunnyk]

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): 7894842

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.