Fix disappearing filtered search in Issues Analytics

What does this MR do and why?

Fixes bug where filtered search disappears when filters have been applied and do not produce any data – it should only ever disappear if a group/project has no issues created over the past twelve months. Also refactors issues_analytics_chart_spec.js a bit.

Screenshots or screen recordings

Before	After
Screen Recording 2023-08-30 at 9.59.24 AM.mov	Screen Recording 2023-08-30 at 11.20.08 AM.mov

How to set up and validate locally

1. Make sure you have a GitLab Ultimate license
2. Go to a group/project with issues, and visit http://{LOCAL_INSTANCE}/groups/{GROUP_PATH}/-/issues_analytics or http://{LOCAL_INSTANCE}/{GROUP_PATH}/{PROJECT_PATH}/-/analytics/issues_analytics
3. Enter one or more filters until they produce no results and the empty state appears.
4. Refresh the page, and verify that the filtered search bar does not disappear.

MR acceptance checklist

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