Creating volume slider haptics view-models on non-empty states.

When the slider state is empty, we don't create a view-model to avoid
erroneous haptics due to an invalid slider range.

Test: manual. Verified that haptics behave correctly when an empty
  slider state is used to compose the UI. Verification via manual
  sensation and via the systemui dumpsys.
Flag: com.android.systemui.haptics_for_compose_sliders
Bug: 376333545
Change-Id: I4c502f9417e2cd73fb8b3f702d2a52fb9b0af788