Currently there is no distinction between null and unset values for BoundStatements: any value that was not set explicitly is considered null.
CASSANDRA-7304 will add support for "unset" values at the protocol level. This can lead to subtle bugs for driver users: if they were relying on the previous behavior ("don't call setter if I want to set my value to null"), the semantics of their queries will now be changed.
Therefore we should force users to be explicit to avoid future ambiguity: if a setter was not called for a bound value, raise an exception when the statement is executed. This check will be removed when the new behavior is introduced.