scripts: utils: migrate_sys_init: handle empty line after ARG_UNUSED

The script left empty lines below ARG_UNUSED. After this patch,

```c
static int f(const struct device *dev)
{
	ARG_UNUSED(dev);

	/* code */
	...
}
```

will become:

```c
static int f(void)
{
	/* code */
	...
}
```

instead of:

```c
static int f(void)
{

	/* code */
	...
}
```

Fixes #56954

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>