CASSANDRA-21625 Column Comparison with null NPEs for Accord Transaction

[alanwang67]

The following transaction NPEs, we should instead reject transactions that compare a column with null. IS NULL/IS NOT NULL should be used instead.

BEGIN TRANSACTION
LET row1 = (SELECT * FROM distributed_test_keyspace.accordtbl0 WHERE k=1);
IF row1.v = null THEN
        UPDATE distributed_test_keyspace.accordtbl0 SET v=1 WHERE k=1;
END IF
COMMIT TRANSACTION

patch by Alan Wang;

reviewed by for CASSANDRA-21625

Co-authored-by: Name1
Co-authored-by: Name2

[dcapwell]

it might be best to do something like value = reference.type().santize(value) or check isNull here... that is just to handle "meaningless emptyness" (legacy concept from thrift days, but it impacts many common types like int)