HPCC-11138 - Fix TopN binaryInsert regression

Introduced regression in way new binaryInsert was used.
Removing last row, didn't reduce row count and left NULL row in
place. Change so that binaryInsert, optionally drops last row.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5635 from jakesmith/hpcc-11138

HPCC-11138 - Fix TopN binaryInsert regression

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>