it's unfortunate they coded it this way as WiFi.begin(_ssid);is the proper way to handle open wireless network.
--> you could edit the library to add one method with the limited signature and duplicate the other code (set _pass to NULL maybe or "") but just call WiFi.begin(_ssid); instead of WiFi.begin(_ssid, _pass);
(there might be other modifications if _pass is used elsewhere - I have not checked)