hide_notification possible bugfix

I'm not sure if this was a bug or I just misunderstood how cheeky should behave, but the description says:
> If no clients are matched it resets to the complete list of clients... and it tells you about it.

Meanwhile, if plugin is used as proposed in README (i.e. without options, just `cheeky.util.switcher()`) then I get `cheeky/util.lua:50: attempt to index upvalue 'options' (a nil value)` and reset fails.