Make splashImage option a `nullOr` type

[ink-pot-monkey]

Currently importing the nixos module with a plain config fails with the error:

❯ sudo nixos-rebuild switch --flake .
building the system configuration...
warning: Git tree '/home/thomassdk/.config/nixos' is dirty
error: A definition for option `boot.loader.grub2-theme.splashImage' is not of type `path'. Definition values:
       - In `/nix/store/y623x85i3kh5r56k0wzx2rq0liprbmy7-source/hosts/stargazer': ""
(use '--show-trace' to show detailed location information)

Config:

grub2-theme = {
  enable = true;
};

This fixes that allowing the configuration to build and use the default splashImage of the theme