Fix a number of strictNullChecks-related issues (#35795)

In preparation to work on enabling
https://www.typescriptlang.org/tsconfig/#strictNullChecks, I fixed all
the issues outside of `web_src` that came up when the option was
enabled. There was also one lint issue in web_src that apparently only
came up with the option enabled, so I fixed that as well.

`isTruthy` is introduced because Typescript has a bug regarding
`filter(Boolean)` which they are seemingly unwilling to fix.

---------

Signed-off-by: silverwind <me@silverwind.io>

web_src/js/features/repo-issue.ts

@@ -261,7 +261,7 @@ export function initRepoPullRequestReview() {
     if (commentDiv) {
       // get the name of the parent id
       const groupID = commentDiv.closest('div[id^="code-comments-"]')?.getAttribute('id');
-      if (groupID && groupID.startsWith('code-comments-')) {
+      if (groupID?.startsWith('code-comments-')) {
         const id = groupID.slice(14);
         const ancestorDiffBox = commentDiv.closest<HTMLElement>('.diff-file-box');