cql3::lists: Fix setter_by_uuid not handing null value
cql3::lists: Fix setter_by_uuid not handing null value

Fixes #6828

When using the scylla list index from UUID extension,
null values were not handled properly causing throws
from underlying layer.

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