_experimental-update-script-combinators.sequence: Loosen attrPath constraint

We only allow a single value for `attrPath` across all sequenced update scripts.
But previously `null` (representing `passthru.updateScript.attrPath` not being defined) was counted as one value.
This would prevent us from explicitly specifying `attrPath` in `gnome.updateScript` in the next commit.

Let’s ignore update scripts without specified `attrPath` attribute for the purpose of this check.

pkgs/common-updater/combinators.nix

@@ -169,9 +169,11 @@ rec {
 
     assert lib.assertMsg (lib.all validateFeatures scripts)
       "Combining update scripts with features enabled (other than “silent” scripts and an optional single script with “commit”) is currently unsupported.";
+
     assert lib.assertMsg (
       builtins.length (
         lib.unique (
+          builtins.filter (attrPath: attrPath != null) (
             builtins.map (
               {
                 attrPath ? null,
@@ -180,7 +182,8 @@ rec {
               attrPath
             ) scripts
           )
-      ) == 1
+        )
+      ) <= 1
     ) "Combining update scripts with different attr paths is currently unsupported.";
 
     {