[Slider] Allow null value

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**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: https://github.com/getodk/collect/pull/6987

Thanks for the consideration!

No files or tests are named in the issue. Start by locating the Android Slider component and its existing value and handle-rendering tests. Confirm the null-value behavior and run the relevant slider tests to verify that the handle is hidden and the track is unshaded.

Written by the indexing model from the issue text.