systemd: minor code cleanup

Trying to make more clear what is happening.

modules/systemd.nix

@@ -46,16 +46,29 @@ let
       filename = "${name}.${style}";
       pathSafeName = mkPathSafeName filename;
 
-      # Needed because systemd derives unit names from the ultimate
-      # link target.
+      # The actual unit content after filtering.
+      finalUnit =
+        let
+          # Filters out fields that are set to `null` or empty list.
+          shouldKeepField =
+            section: key: value:
+            value != null && value != [ ];
+
+          # Filters out empty sections.
+          shouldKeepSection = _: value: value != { };
+
+          inherit (lib) mapAttrs filterAttrs;
+
+          filteredFields = mapAttrs (section: filterAttrs (shouldKeepField section)) serviceCfg;
+          filteredSections = filterAttrs shouldKeepSection filteredFields;
+        in
+        filteredSections;
+
+      # Needed because systemd derives unit names from the ultimate link target.
       source =
         pkgs.writeTextFile {
           name = pathSafeName;
-          text = toSystemdIni (
-            lib.filterAttrs (_: v: v != { }) (
-              lib.mapAttrs (_: lib.filterAttrs (_: v: v != null && v != [ ])) serviceCfg
-            )
-          );
+          text = toSystemdIni finalUnit;
           destination = "/${filename}";
         }
         + "/${filename}";