Handle DOWN_KEYCODE with no results

If, when searching, one pressed the down arrow key when there were no
results, this caused an uncaught exception and defocused the search
box.

Let's prevent this and keep the search box focused when pressing down
in this state by checking first whether there is a result for us to
focus instead.

Author: traviscross

src/front-end/searcher/searcher.js

@@ -369,8 +369,11 @@ window.search = window.search || {};
             searchbar.select();
         } else if (hasFocus() && e.keyCode === DOWN_KEYCODE) {
             e.preventDefault();
-            unfocusSearchbar();
-            searchresults.firstElementChild.classList.add('focus');
+            const first = searchresults.firstElementChild;
+            if (first !== null) {
+                unfocusSearchbar();
+                first.classList.add('focus');
+            }
         } else if (!hasFocus() && (e.keyCode === DOWN_KEYCODE
                                 || e.keyCode === UP_KEYCODE
                                 || e.keyCode === SELECT_KEYCODE)) {