ci/eval/compare: handle missing packages

Not all packages that are reported as changed will actually exist on the
platform that the maintainers are colleted on.

This is the case for some attributes that are only available on Darwin
or explicitly set to `null` on Linux. By filtering out packages without
maintainers, these are ignored - and we should potentially get a small
performance improvement as well.

ci/eval/compare/maintainers.nix

@@ -22,18 +22,27 @@ let
 
   anyMatchingFiles = files: builtins.any anyMatchingFile files;
 
-  attrsWithMaintainers = builtins.map (
-    name:
-    let
-      package = lib.getAttrFromPath (lib.splitString "." name) pkgs;
-    in
-    {
-      inherit name package;
-      # TODO: Refactor this so we can ping entire teams instead of the individual members.
-      # Note that this will require keeping track of GH team IDs in "maintainers/teams.nix".
-      maintainers = package.meta.maintainers or [ ];
-    }
-  ) (changedattrs ++ removedattrs);
+  attrsWithMaintainers = lib.pipe (changedattrs ++ removedattrs) [
+    (builtins.map (
+      name:
+      let
+        # Some packages might be reported as changed on a different platform, but
+        # not even have an attribute on the platform the maintainers are requested on.
+        # Fallback to `null` for these to filter them out below.
+        package = lib.attrByPath (lib.splitString "." name) null pkgs;
+      in
+      {
+        inherit name package;
+        # TODO: Refactor this so we can ping entire teams instead of the individual members.
+        # Note that this will require keeping track of GH team IDs in "maintainers/teams.nix".
+        maintainers = package.meta.maintainers or [ ];
+      }
+    ))
+    # No need to match up packages without maintainers with their files.
+    # This also filters out attributes where `packge = null`, which is the
+    # case for libintl, for example.
+    (builtins.filter (pkg: pkg.maintainers != [ ]))
+  ];
 
   relevantFilenames =
     drv: