cql3: expr: do not allow unset values inside collections
cql3: expr: do not allow unset values inside collections

Semantic of unset values inside collections is undefined.
Previous behavior of transforming list with unset value into unset value
was removed, because I couldn't find a reason for its existence.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla