search: hide empty categories by default

The search page (https://git-scm.com/search/results?search=something) is
now backed by Pagefind, which allows specifying a "category", i.e.
whether to show only results from the book or the reference, etc.

By default, the Pagefind UI shows all categories, even if there are no
matches for the current search term(s) in a given category.

Let's turn that off.

See https://pagefind.app/docs/ui/#show-empty-filters for more details.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

assets/js/application.js

@@ -362,6 +362,7 @@ var Search = {
 
     const ui = new PagefindUI({
       element: "#search-div",
+      showEmptyFilters: false,
       showSubResults: true,
       showImages: false,
       language,