workload/schemachanger: fix trigger function generation

Previously, the triggers we generated on tables would silently discard
IUD operations. This patch makes sure that the trigger does random work
and allows the IUD operation to continue.

Fixes: #150752

Release note: None

Author: fqazi

pkg/workload/schemachange/operation_generator.go

@@ -5398,8 +5398,9 @@ func (og *operationGenerator) createTrigger(ctx context.Context, tx pgx.Tx) (*op
 	if err != nil {
 		return nil, err
 	}
-
-	triggerFunction := fmt.Sprintf(`CREATE FUNCTION %s() RETURNS TRIGGER AS $FUNC_BODY$ BEGIN %s;RETURN NULL;END; $FUNC_BODY$ LANGUAGE PLpgSQL`, triggerFunctionName, selectStmt.sql)
+	// Our trigger function will always return the original value to avoid
+	// breaking inserts.
+	triggerFunction := fmt.Sprintf(`CREATE FUNCTION %s() RETURNS TRIGGER AS $FUNC_BODY$ BEGIN %s;RETURN NEW;END; $FUNC_BODY$ LANGUAGE PLpgSQL`, triggerFunctionName, selectStmt.sql)
 
 	og.LogMessage(fmt.Sprintf("Created trigger function %s", triggerFunction))