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/features/copycontent.ts

@@ -20,7 +20,7 @@ export function initCopyContent() {
       btn.classList.add('is-loading', 'loading-icon-2px');
       try {
         const res = await GET(rawFileLink, {credentials: 'include', redirect: 'follow'});
-        const contentType = res.headers.get('content-type');
+        const contentType = res.headers.get('content-type')!;
 
         if (contentType.startsWith('image/') && !contentType.startsWith('image/svg')) {
           isRasterImage = true;