`nvbootctrl` tries to read a missing `/etc/nv_boot_control.conf` (and outputs confusing messages)

[nagisa]

When running commands such as nvbootctrl get-current-slot, the output contains messages like these:

Error: null input file!

which can be somewhat confusing. From strace it would seem that this is largely caused by a missing /etc/nv_boot_control.conf file. Maybe if we added it (even if an empty one), the error would go away?