HPCC-8748 Start treating constant rows as link counted

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #3962 from ghalliday/issue8748

HPCC-8748 Start treating constant rows as link counted

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>