Motivation for this change

In order for the grub interface to function properly on the serial console provided by some ARM/aarch64 hardware, the background image and font functionality of Grub must not be used.

The grub module's font type supports being nullable, however the code utilizing the font variable did not accept the null option.

The first commit should be backported, fixing the null option. This will mean that a server where the font has always been null, no font will be loaded.

The second commit would be nice to backport, but I would understand if there is hesitation. This commit would fix systems where a font has been copied in to place, but is no longer desirable after setting font to null.

This behavior is modeled after how the background image is handled. Ideally we would also remove the font / background image, but I'm not sure that should be in scope for this fix.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

cc @dezgeg