HPCC-8772 Fix core when serializing a row that didn't need it

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #3945 from ghalliday/issue8772

HPCC-8772 Fix core when serializing a row that didn't need it

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>