nixos/wireguard-networkd: only set PresharedKey credential when given a file

This patch fixes an oversight in the initial implementation
of using systemd credentials in 6bc8dcc630
that would cause a warning to be logged about a missing credential
when presharedKeyFile wasn't set.

nixos/modules/services/networking/wireguard-networkd.nix

@@ -61,7 +61,8 @@ let
     interfaceName: peer:
     removeNulls {
       PublicKey = peer.publicKey;
-      PresharedKey = "@${presharedKeyCredential interfaceName peer}";
+      PresharedKey =
+        if peer.presharedKeyFile == null then null else "@${presharedKeyCredential interfaceName peer}";
       AllowedIPs = peer.allowedIPs;
       Endpoint = peer.endpoint;
       PersistentKeepalive = peer.persistentKeepalive;