Add missing schedules empty state unit coverage

What does this MR do and why?

I noticed we do not have a spec for our pipeline schedules empty state component. But we do have coverage in spec/frontend/ci/pipeline_schedules/components/pipeline_schedules_spec.js

This MR adds a unit test for ~/ci/pipeline_schedules/components/pipeline_schedules_empty_state.vue

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Make sure pipeline passes
2. Make sure I'm not crazy and a spec actually does exist somewhere

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #416089 (closed)