HPCC-8971 Optimize SEQUENTIAL of a single action.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4101 from ghalliday/issue8971

HPCC-8971 Optimize SEQUENTIAL of a single action.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>