fix error when trying to get GPU percentage, but GPU is detected

src/Resources/Resources.vala

@@ -86,7 +86,7 @@ public class Monitor.Resources : Object {
                    swap_total = swap.total,
                    network_up = network.bytes_out,
                    network_down = network.bytes_in,
-                   gpu_percentage = gpu.percentage
+                   gpu_percentage = gpu != null ? gpu.percentage : 0
         };
     }
 }