Add empty state to pending members page

What does this MR do and why?

The MR adds an empty state to the pending members page when there are no pending members left to approve.

note: no changelog since the change is behind a feature flag.

Related to #438719 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable saas_user_caps feature flag
2. Run GDK in SaaS mode
3. As admin/owner, navigate to a group's members page (Group page > Manage > Members)
4. Using the alert in the members page, navigate to the pending members page
5. If the page is empty, you should see the empty state introduced by this MR (otherwise, approve all pending members to see the empty state)