Prevent collapsible fieldset from showing when facet is empty

This is really a BEF bug: 🐛 Collapsible details element not hidden if "Hide filter, if no options" is selected. Active

But we can implement a workaround for now.

Accidentally referenced the BEF issue in my commit message, but here's the code: https://git.drupalcode.org/project/bef_links_filter/-/commit/500cbac2674...