fix: do not ignore 0 value prop

[vonovak]

Summary:

currentValue is used when rendering StepMarker and currently it doesn't seem to treat 0 correctly.

Test Plan:

  const renderStepMarker = React.useCallback(({ stepMarked, currentValue }: MarkerProps) => {
    return stepMarked ? <Text>{currentValue}</Text> : null;
  }, []);

<Slider
  minimumValue={-3}
  value={0}
  maximumValue={5}
  onValueChange={setValue}
  StepMarker={renderStepMarker}
/>

fixed

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