allow null tailscaleAuthKeyFile

shooting blanks for testing, not sure if this will work

Author: supermarin

flake.nix

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