Fix user cap alert on non-namespace pages

What does this MR do and why?

When a page is not within a namespace and we try to display the namespace user cap alert, we would receive an error for building the pending_members_group_usage_quotas_path.

We need to use .persisted? because by default, we build a new Namespace record and pass it along to the frontend.

Screenshots or screen recordings

How to set up and validate locally

1. export GITLAB_SIMULATE_SAAS=1
2. gdk start
3. Enable free_user_cap as feature flag globally

   bin/rails c
   Feature.enable(:free_user_cap)

4. Go to http://localhost:3000/groups/new
5. See the error (without this fix)

Reason is that we reuse the user_cap_available?

MR acceptance checklist

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