Add empty state to the pipeline schedules page

What does this MR do and why?

Adds an empty state to the pipelines schedules page behind the pipeline_schedules_vue feature flag

#376187 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the pipeline_schedules_vue feature flag in the console

   echo "Feature.enable(:pipeline_schedules_vue)" | bundle exec rails c

2. Visit any project and go to the pipeline schedules page

3. Make sure not to have any pipeline schedules created and the empty state will show

MR acceptance checklist

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