return the result on transaction (#1725)

return the result on transaction

Reviewed-on: https://gitea.com/xorm/xorm/pulls/1725

engine.go

@@ -1263,11 +1263,11 @@ func (engine *Engine) Transaction(f func(*Session) (interface{}, error)) (interf
 
 	result, err := f(session)
 	if err != nil {
-		return nil, err
+		return result, err
 	}
 
 	if err := session.Commit(); err != nil {
-		return nil, err
+		return result, err
 	}
 
 	return result, nil