Altered test to expect error on UPDATE SET <not_null_col> = NULL instead
of expecting warning and re-recorded .result file.


mysql-test/r/csv_not_null.result:
  Bug#34468 - csv_not_null fails
  Altered test to expect error on UPDATE SET <not_null_col> = NULL instead
  of expecting warning and re-recorded .result file.  New results file passes
mysql-test/t/csv_not_null.test:
  Bug#34468 - csv_not_null fails
  Altered test to expect error on UPDATE SET <not_null_col> = NULL instead
  of expecting warning and re-recorded .result file.  Also cleaned up comments