grub: Fix incorrect upgrade to new `jsonStateLine`. See #85895. (#92520)

Fixes error

    Can't use an undefined value as an ARRAY reference at /nix/store/...-install-grub.pl line 642, <FILE> line 5.

with `/boot/grub/state` being:

```
grub
2.04
no
/dev/sda
/boot

```

I am not sure where the trailing empty line can come from; the script does not
seem to write it. In any case, now we handle that situation as well.

Further, ensure that `extraGrubInstallArgs` defaults to the empty array
if its key is not present in the `jsonState`.