Describe the bug

If you set isoImage.grubTheme to null, it fails to build an ISO.

Steps To Reproduce

create a basic configuration.nix for an installer with isoImage.grubTheme set to null

error: cannot coerce null to a string

       at /nix/store/ha4ld6wd29q1igjmhjdvkvbxpxxxxbxf-source/nixos/modules/installer/cd-dvd/iso-image.nix:255:5:

          254|     strictDeps = true;
          255|   } ''
             |     ^
          256|     mkdir -p $out/EFI/boot/

       … while evaluating the attribute 'buildCommand' of the derivation 'efi-directory'

Expected behavior

Builds image not using theme and embedding spashImage specifried

Screenshots

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Additional context

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Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
output here