[TSVB] fix point visibility regression (elastic#202358)

## Summary

fix elastic#200255

The regression was caused by a [breaking
change](elastic/elastic-charts#2525) introduced
by elastic-charts around the `area.points.visible` style, that passed
from a `boolean` to a union of `'always' | 'never' | 'auto'`;

Author: markov00

src/plugins/vis_types/timeseries/public/application/visualizations/views/timeseries/decorators/__snapshots__/area_decorator.test.js.snap

@@ -27,7 +27,7 @@ export const getAreaStyles = ({ points, lines, color }) => ({
       radius: points.radius || 0.5,
       stroke: color || DEFAULT_COLOR,
       strokeWidth: points.lineWidth || 5,
-      visible: points.lineWidth > 0 && Boolean(points.show),
+      visible: points.lineWidth > 0 && Boolean(points.show) ? 'always' : 'never',
     },
   },
   curve: lines.steps ? CurveType.CURVE_STEP_AFTER : CurveType.LINEAR,