Fix empty state pipelines page

What does this MR do and why?

When the runners_availability_section experiment was active but rendering the control variant, the empty state pipelines page would render no templates. This fixes that bug.

Screenshots or screen recordings

When runners_availability_section experiment is active and the experiment subject in the control variant:

before	after

How to set up and validate locally

1. Find a local project and its group
2. Enable the experiment in the control state for the group by executing the following commands:

• include Gitlab::Experiment::Dsl
• Feature.enable(:runners_availability_section, experiment(:runners_availability_section, :control, namespace: group))

1. Visit http://localhost:3000/{group}/{project}/-/pipelines

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.