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 <[email protected]>
Signed-off-by: wxiaoguang <[email protected]>
Co-authored-by: delvh <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>

web_src/js/features/comp/SearchUserBox.ts

@@ -17,7 +17,7 @@ export function initCompSearchUserBox() {
       url: `${appSubUrl}/user/search_candidates?q={query}&orgs=${includeOrgs}`,
       onResponse(response: any) {
         const resultItems = [];
-        const searchQuery = searchUserBox.querySelector('input').value;
+        const searchQuery = searchUserBox.querySelector('input')!.value;
         const searchQueryUppercase = searchQuery.toUpperCase();
         for (const item of response.data) {
           const resultItem = {