FE: provide empty state for plugin picker

[TimBroddin]

In the create/edit schedule, let's provide an empty state for the plugin list view.

cc @JanaMW27

Context: #88376 (comment)

[JanaMW27]

hummm... is this in case the user doesn't have any plugin?
if that is the case they should not be able to access the feature no?

[TimBroddin]

It's the case where the user doesn't have any plugin that's not managed by us. We currently show a banner in the add/edit view when this happens:

It would be nice if we also show something inside the empty list view there.

[JanaMW27]

My take on this:

• If no plugins can be included in an update schedule, the user shouldn't be able to access the feature.
• If for some reason they still are able to access it, I would have an announcement on the empty schedules page saying: To set up schedules to manage your plugins, first you need to install plugins that are not managed by the plugin provider.
• I would not have the Create a new schedule CTA and I would replace it with one that would take the user to the plugins page (Explore plugins)
• I would not allow the user to access the New Schedule page
  

Is there a specific case scenario where they may still have access to a Schedule page even though no plugins are available?