HPCC-12110 MySQL plugin returns random data for NULL fields

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6358 from richardkchapman/mysql-nulls

HPCC-12110 MySQL plugin returns random data for NULL fields

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>