HPCC-12122 Always pass allocator even for null records

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6639 from ghalliday/issue12122

HPCC-12122 Always pass allocator even for null records

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>