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`.
e8c1ff9ef8