nixos/uki: allow deviceTree to be excluded from UKI

When setting `hardware.deviceTree.name` and `.enable`, a DeviceTree
section is added to UKI. This is nice default behavior for arm64
systems. When `hardware.deviceTree.package` plumbs into UEFI firmware
(e.g. u-boot), the UKI replaces the UEFI-supplied DTB. Besides bloating
the UKI, this has the side-effect of preventing UEFI firmware from
making modifications to the devicetree.

Filter out null boot.uki.settings values so that a NixOS configuration
can set `boot.uki.settings.UKI.DeviceTree = null;` to remove the default
behavior of packing the dtb into the UKI.

(cherry picked from commit e949e86001)

nixos/modules/system/boot/uki.nix

@@ -12,6 +12,8 @@ let
   inherit (pkgs.stdenv.hostPlatform) efiArch;
 
   format = pkgs.formats.ini { };
+
+  ukiSettings = lib.filterAttrsRecursive (_: v: v != null) cfg.settings;
 in
 
 {
@@ -94,7 +96,7 @@ in
           };
     };
 
-    boot.uki.configFile = lib.mkOptionDefault (format.generate "ukify.conf" cfg.settings);
+    boot.uki.configFile = lib.mkOptionDefault (format.generate "ukify.conf" ukiSettings);
 
     system.boot.loader.ukiFile =
       let