HPCC-12297 Remove use of NULL for a non-pointer comparison

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6491 from ghalliday/issue12297

HPCC-12297 Remove use of NULL for a non-pointer comparison

Reviewed-By: Russ Whitehead <william.whitehead@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>