fix: consider 0 value for `pageX`

I was getting the error `Uncaught TypeError: Cannot read property '0' of undefined` similar to #57, #73, and #77. It seems like the `||` in this [line](https://github.com/andreruffert/rangeslider.js/blob/develop/src/rangeslider.js#L253) is counting `0` as falsy, so it keeps going until it gets to `e.originalEvent.touches[0].clientX` where it raises an error. I changed to explicitly checking that the values are `undefined`, and it fixed the errors for me.