cql3: Don't allow unset values inside UDT
cql3: Don't allow unset values inside UDT

Scylla doesn't support unset values inside UDT.
The old code used to convert unset to null, which seems incorrect.

There is an extra space in the error message to retain compatability with Cassandra.

Signed-off-by: Jan Ciolek <b66a6f3989e26cb552db14b7ab163c3a50292e24@scylladb.com>

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