Fix MERGE command tag for actions blocked by BEFORE ROW triggers.

Fix MERGE command tag for actions blocked by BEFORE ROW triggers.

This ensures that the row count in the command tag for a MERGE is
correctly computed in the case where UPDATEs or DELETEs are skipped
due to a BEFORE ROW trigger returning NULL (the INSERT case was
already handled correctly by ExecMergeNotMatched() calling
ExecInsert()).

Back-patch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCU8XEmR0JWKDtyb7iZ%3DqCffxS9uyJt0iOZ4TV4RT%2Bow1w%40mail.gmail.com