grub2-15_ostree: Graceful exit if /etc/default/grub doesn't exist

grub2-15_ostree: Graceful exit if /etc/default/grub doesn't exist

With the new bootupd installation path in Anaconda, the
`/etc/default/grub` config file is not written anymore as we are only
using BLS configs with new enough bootloaders.

We thus don't need to generate (duplicated) legacy boot entries.

We still need to keep this logic in place in Atomic Desktops
(Silverblue, etc.) until we've actually landed bootupd there and forced
a bootloader update for everybody.

See: https://github.com/fedora-silverblue/issue-tracker/issues/530
See: https://github.com/fedora-silverblue/issue-tracker/issues/120
See: https://fedoraproject.org/wiki/Changes/FedoraSilverblueBootupd