Fix type coercion in diff viewer

The first file has rank 0, which by type coercion is false and this was
not downloadable after switching the tab to another file and back.

Author: Kevinjil

webapp/public/js/domjudge.js

@@ -1340,7 +1340,7 @@ function initDiffEditor(editorId) {
     const download = wrapper.find(".download")[0];
     const edit = wrapper.find(".edit")[0];
     const updateTabRank = (rank) => {
-        if (rank) {
+        if (rank !== undefined) {
             let url = new URL(download.href);
             url.searchParams.set("fetch", rank);
             enableButton(download, url);