Fix navigation JS error (#3655)

Before, the UP navigation button was generating an error when no article was
selected. The process to navigate to the first element was using the position
of the current element to compute the offset, thus generating said error.
Now, if no article is selected, the first article is used as a fall-back.

See #3654

p/scripts/main.js

@@ -1093,7 +1093,7 @@ function init_nav_entries() {
 				return false;
 			};
 		nav_entries.querySelector('.up').onclick = function (e) {
-				const active_item = document.querySelector('.flux.current'),
+				const active_item = (document.querySelector('.flux.current') || document.querySelector('.flux')),
 					windowTop = document.scrollingElement.scrollTop,
 					item_top = active_item.offsetParent.offsetTop + active_item.offsetTop;