[TSVB] Use the empty label for / terms (#154647)

## Summary

Closes #150993

In case there is a single `/` in the markdown it uses the (empty) label
as we are doing in other cases.

<img width="808" alt="image"
src="https://user-images.githubusercontent.com/17003240/230896158-fbdb2f73-76c3-44a0-8e69-041dadd379a6.png">


### 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/vis_types/timeseries/common/empty_label.test.ts

@@ -0,0 +1,27 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import { getValueOrEmpty } from './empty_label';
+
+describe('getValueOrEmpty', () => {
+  test('returns the value if not empty or slash value given', () => {
+    expect(getValueOrEmpty('/test/blog')).toEqual('/test/blog');
+  });
+
+  test('returns (empty) if value is slash', () => {
+    expect(getValueOrEmpty('/')).toEqual('(empty)');
+  });
+
+  test('returns (empty) if value is empty', () => {
+    expect(getValueOrEmpty('')).toEqual('(empty)');
+  });
+
+  test('returns (empty) if value is null', () => {
+    expect(getValueOrEmpty(null)).toEqual('(empty)');
+  });
+});

src/plugins/vis_types/timeseries/common/empty_label.ts

@@ -13,7 +13,7 @@ export const emptyLabel = i18n.translate('visTypeTimeseries.emptyTextValue', {
 });
 
 export const getValueOrEmpty = (value: unknown) => {
-  if (value === '' || value === null || value === undefined) {
+  if (value === '' || value === '/' || value === null || value === undefined) {
     return emptyLabel;
   }
   return `${value}`;