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-settings.ts

@@ -101,7 +101,7 @@ function initRepoSettingsBranches() {
   // show the `Matched` mark for the status checks that match the pattern
   const markMatchedStatusChecks = () => {
     const patterns = (document.querySelector<HTMLTextAreaElement>('#status_check_contexts').value || '').split(/[\r\n]+/);
-    const validPatterns = patterns.map((item) => item.trim()).filter(Boolean);
+    const validPatterns = patterns.map((item) => item.trim()).filter(Boolean as unknown as <T>(x: T | boolean) => x is T);
     const marks = document.querySelectorAll('.status-check-matched-mark');
 
     for (const el of marks) {