Adjust search results title when no results exist

Update results title to display "0 Results" if no results are returned
on submitting the form.
Currently if the form is submitted without typing anything in, the
search results title says "Loading search index".

lib/assets/javascripts/_modules/search.js

@@ -69,7 +69,14 @@
       $searchForm.on('submit', function (e) {
         e.preventDefault()
         showResults()
-        $searchResults.find('.search-result__title a').first().focus()
+        var $firstResult = $searchResults.find('.search-result__title a').first()
+        if ($firstResult.length) {
+          $firstResult.focus()
+        } else {
+          // if there are no results, show the "0 results" state
+          results = []
+          updateTitle()
+        }
       })
 
       // Closing the search results, move focus back to the search input