[FIX] hw_scanner: iterate on empty list

Avoid error None is not iterable when no device is connected

addons/hw_scanner/controllers/main.py

@@ -120,10 +120,10 @@ class Scanner(Thread):
     def get_devices(self):
         try:
             if not evdev:
-                return None
+                return []
 
             if not os.path.isdir(self.input_dir):
-                return None
+                return []
 
             new_devices = [device for device in listdir(self.input_dir)
                            if join(self.input_dir, device) not in [dev.evdev.fn for dev in self.open_devices]]