[XY] Fixes the threshold visibility (#158917)

## Summary

Closes https://github.com/elastic/kibana/issues/158343

Fixes the broken reference line in aggbased xy charts.

<img width="1715" alt="image"
src="ae48f0b8-475a-44e2-866a-c56e9da8e8b0">

The problem was the default here, apparently the expressions fail when
the default is an empty string. The expression was failing and as a
result the reference line was never rendered

### Checklist
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

src/plugins/chart_expressions/expression_xy/common/expression_functions/reference_line.ts

@@ -108,7 +108,6 @@ export const referenceLineFunction: ReferenceLineFn = {
     forAccessor: {
       types: ['string'],
       help: '',
-      default: '',
     },
   },
   fn(table, args) {

test/functional/page_objects/visualize_chart_page.ts

@@ -248,6 +248,12 @@ export class VisualizeChartPageObject extends FtrService {
     return items.map(({ name }) => name);
   }
 
+  public async getReferenceLine(selector: string) {
+    const items = (await this.getEsChartDebugState(selector))?.annotations;
+    const referenceLine = items?.filter(({ type }) => type === 'line');
+    return referenceLine;
+  }
+
   public async getLegendEntries() {
     const isVisTypePieChart = await this.isNewLibraryChart(partitionVisChartSelector);
     const isVisTypeHeatmapChart = await this.isNewLibraryChart(heatmapChartSelector);

x-pack/test/functional/apps/lens/open_in_lens/agg_based/xy.ts

@@ -205,6 +205,8 @@ export default function ({ getPageObjects, getService }: FtrProviderContext) {
       await visEditor.clickOptionsTab();
       await visEditor.toggleShowThresholdLine();
       await visEditor.clickGo(isNewChartsLibraryEnabled);
+      const line = await visChart.getReferenceLine('xyVisChart');
+      expect(line?.length).to.be(1);
       await header.waitUntilLoadingHasFinished();
 
       await visualize.navigateToLensFromAnotherVisulization();