go/printer: incorrect formatting of empty comments

At tip (but not 1.18.2), go/printer incorrectly removes the blank line between `A` and `B` here:

```
func A() {
}

//
func B() {
}
```

resulting in

```
func A() {
}
func B() {
}
```

https://go.dev/play/p/VTb8TuGHV2U?v=gotip