naughty: return nothing, not nil

This is not a valid return value for a D-Bus method.

Signed-off-by: Julien Danjou <julien@danjou.info>

lib/naughty.lua.in

@@ -464,7 +464,7 @@ if capi.dbus then
             if title ~= "" then
                 args.text = title
             else
-                return nil
+                return
             end
         end
         local score = 0