nixos/networkmanager: fix serializing an invalid `wifi.powersave=null`

NetworkManager does not define `null` as a valid value for `wifi.powersave`, and while strangely it doesn't seem to report any errors upon encountering it, it's a weird value and shouldn't be here. This is actually very similar to how the module used to work before the refactor in https://github.com/NixOS/nixpkgs/pull/118308 (see this [snippet](https://github.com/NixOS/nixpkgs/pull/118308/files#diff-0a708e7b053cf5df7620b5262936553af2242d2ce9dabde5bbeba221ece0a021L45-L46) from the PR changes).

nixos/modules/services/networking/networkmanager.nix

···
         networkmanager.connectionConfig = {
           "ethernet.cloned-mac-address" = cfg.ethernet.macAddress;
           "wifi.cloned-mac-address" = cfg.wifi.macAddress;
-          "wifi.powersave" =
-            if cfg.wifi.powersave == null then
-              null
-            else if cfg.wifi.powersave then
-              3
-            else
-              2;
+          "wifi.powersave" = lib.mkIf (cfg.wifi.powersave != null) (if cfg.wifi.powersave then 3 else 2);
         };
       }
     ];