Allow null tailscaleAuthKeyFile

Since systemd service handles optional tailscaleAuthKeyFile,
allow for optional declaration in the module.

Signed-off-by: Marin <[email protected]>

Author: supermarin

flake.nix

@@ -94,7 +94,8 @@
           };
 
           tailscaleAuthKeyFile = mkOption {
-            type = types.path;
+            type = types.nullOr types.path;
+            default = null;
             description = "Path to file containing the Tailscale Auth Key";
           };