Fix MERGE tuple count with DO NOTHING

Reporting tuples for which nothing is done is useless and goes against
the documented behavior, so don't do it.

Backpatch to 15.

Reported by: Luca Ferrari
Discussion: https://postgr.es/m/CAKoxK+42MmACUh6s8XzASQKizbzrtOGA6G1UjzCP75NcXHsiNw@mail.gmail.com

src/backend/executor/nodeModifyTable.c

@@ -2799,7 +2799,7 @@ lmerge_matched:
 		{
 			case TM_Ok:
 				/* all good; perform final actions */
-				if (canSetTag)
+				if (canSetTag && commandType != CMD_NOTHING)
 					(estate->es_processed)++;
 
 				break;