search: avoid reacting to an empty search term

When someone currently presses the `/` key to let the search box get the
focus, and then immediately presses the Enter key without typing a
search term, the site currently redirects to
https://git-scm.com/undefined.

The same happens when the search term consists of only one character.

That's because it is somewhat unexpected that anybody would want to see
_all_ search results before seeing _some_ search results.

Let's fix that.

Incidentally, this also helps with adding a test that verifies that
hitting Enter in the search box will never direct the browser to that
`/undefined` URL, no matter how quickly that search term and the Enter
key are provided: The `runSearch()` function triggers on key-up, which
Playwright's `fill()` method (which can be used to fill in a search
term) does not emit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

assets/js/application.js

@@ -371,6 +371,12 @@ var Search = {
   selectResultOption: function() {
     var link = $('#search-results a')[Search.selectedIndex];
     var url = $(link).attr('href');
+    if (!url) {
+      const term = $('#search-text').val();
+      if (!term) return;
+      const language = document.querySelector("html")?.getAttribute("lang");
+      url = `${baseURLPrefix}search/results?search=${term}${language && `&language=${language}`}`;
+    }
     window.location.href = url;
     selectedIndex = 0;
   },