Show runner stats only when some runners are found

What does this MR do and why?

This change adds an extra check before showing the runner stats to avoid showing empty stats when no runners are found.

Changelog: changed

Screenshots or screen recordings

before	after

This change does not affect the UI when some runners are found:

How to set up and validate locally

1. Visit Admin -> Runners
2. Select a filter that yields no results
   • On my local I use Paused: Yes for example.
3. Confirm the stats (Online, Offline, Stale, ...) at the top of the table are not shown.

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 #377541 (closed)