Classwriter should check if columnNames generated are null before accessing them

If a table does not at the time of DROP, Netezza throws an exception but also sets the transaction as ignored (all changes through the connection will be ignored until the txn is either rollback or committed. While adding more tests, I missed the rollback and the create table was ignored, and subsequently ClassWriter.cleanColNames() threw NPE. I am fixing Netezza tests also, but we should check for this and throw a IOException instead of failing with NPE