[Slider] Allow null value

[lognaturel]

Is your feature request related to a problem? Please describe.
We don't want the slider to always have a selected value -- we're using it in a context where the end user could choose to leave it blank.

Describe the solution you'd like
When the slider has a null value, don't display the handle. Display the slider itself as though the bottom of the range were selected (none of it shaded in).

Describe alternatives you've considered
We previously extended the slider to hide the handle when the backing value is null but it's a brittle approach that broke with the latest version of the component: getodk/collect#6987

Thanks for the consideration!