Enable TypeScript strictNullChecks (#35843)

A big step towards enabling strict mode in Typescript.

There was definitely a good share of potential bugs while refactoring
this. When in doubt, I opted to keep the potentially broken behaviour.
Notably, the `DOMEvent` type is gone, it was broken and we're better of
with type assertions on `e.target`.

---------

Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/components/DashboardRepoList.vue

@@ -110,9 +110,9 @@ export default defineComponent({
   },
 
   mounted() {
-    const el = document.querySelector('#dashboard-repo-list');
+    const el = document.querySelector('#dashboard-repo-list')!;
     this.changeReposFilter(this.reposFilter);
-    fomanticQuery(el.querySelector('.ui.dropdown')).dropdown();
+    fomanticQuery(el.querySelector('.ui.dropdown')!).dropdown();
 
     this.textArchivedFilterTitles = {
       'archived': this.textShowOnlyArchived,