Allow using an empty string for the `onText` and `offText` attributes

Previous code didn't allow you to use a ```""``` as the value for ```onText``` or ```offText``` because ```""``` is _falsy_, and you got the default "on" and "off" values instead, which is quite unexpected for a user.

Even the "simple switch" example has to resort to the hack of using a one-space string ```" "``` for not showing text, so I think this should be useful :-)