fix exponentialRampToValueAtTime to 0 issue

The exponentialRampToValueAtTime() method doesn't support values which less or equal to 0.

From the [example on MDN](https://developer.mozilla.org/en-US/docs/Web/API/AudioParam.exponentialRampToValueAtTime#Examples) :
> Note: A value of 0.01 was used for the value to ramp down to in the last function rather than 0, as an invalid or illegal string error is thrown if 0 is used — the value needs to be positive.