nixos/repart-image: don't pass seed when it's null, update documentation

From systemd-repart(8):

If not specified it is attempted to read the machine ID from the host
(or more precisely, the root directory configured via --root=) and use
it as seed instead, falling back to a randomized seed otherwise. Use
--seed=random to force a randomized seed.

The option documentation now suggests the special value "random", for
a random seed rather than relying on the implicit fact that there is
no machine ID available in the build sandbox, causing repart to fallback
on generating a random seed.

Fixes #419331

nixos/modules/image/repart-image.nix

@@ -173,11 +173,13 @@ stdenvNoCC.mkDerivation (
         "--architecture=${systemdArch}"
         "--dry-run=no"
         "--size=auto"
-        "--seed=${seed}"
         "--definitions=${finalAttrs.finalRepartDefinitions}"
         "--split=${lib.boolToString split}"
         "--json=pretty"
       ]
+      ++ lib.optionals (seed != null) [
+        "--seed=${seed}"
+      ]
       ++ lib.optionals createEmpty [
         "--empty=create"
       ]

nixos/modules/image/repart.nix

@@ -161,8 +161,9 @@ in
       # Generated with `uuidgen`. Random but fixed to improve reproducibility.
       default = "0867da16-f251-457d-a9e8-c31f9a3c220b";
       description = ''
-        A UUID to use as a seed. You can set this to `null` to explicitly
+        A UUID to use as a seed. You can set this to `random` to explicitly
         randomize the partition UUIDs.
+        See {manpage}`systemd-repart(8)` for more information.
       '';
     };