Explore groups - hide alert when empty state is shown

On https://gitlab.com/explore/groups this alert is shown when this list is empty and that doesn't make sense.

We should only show this alert when the list is populated with groups.

Implementation guide

1. Move the alert to Vue and use GlAlert. We will need to figure out a way to put it in app/assets/javascripts/groups/components/app.vue#L258 or a component that wraps that.
2. Check if the list is empty and hide the alert