I just tried with the latest emacs-29 branch, and found that the c-mode 

and c-ts-mode are the same when starting from "emacs -Q".

After RET in both c-mode and c-ts-mode, I got:

```

if (a) {

|}

```

So, extra line might be a feature of a minor mode instead of major c-mode

or c-ts-mode where I was wrong. Then I found that this is a feature of the 

package 'smartparens' . I'm trying to find why it does not work in c-ts-mode.