Merge pull request #264811 from emilylange/systemd-empty-unit-build

nixos/systemd-lib: fix building of empty unit files

nixos/lib/systemd-lib.nix

@@ -20,7 +20,10 @@ in rec {
       pkgs.runCommand "unit-${mkPathSafeName name}"
         { preferLocalBuild = true;
           allowSubstitutes = false;
-          inherit (unit) text;
+          # unit.text can be null. But variables that are null listed in
+          # passAsFile are ignored by nix, resulting in no file being created,
+          # making the mv operation fail.
+          text = optionalString (unit.text != null) unit.text;
           passAsFile = [ "text" ];
         }
         ''