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>

tests/e2e/utils_e2e.ts

@@ -33,15 +33,15 @@ export async function login_user(browser: Browser, workerInfo: WorkerInfo, user:
 }
 
 export async function load_logged_in_context(browser: Browser, workerInfo: WorkerInfo, user: string) {
-  let context;
   try {
-    context = await browser.newContext({storageState: `${ARTIFACTS_PATH}/state-${user}-${workerInfo.workerIndex}.json`});
+    return await browser.newContext({storageState: `${ARTIFACTS_PATH}/state-${user}-${workerInfo.workerIndex}.json`});
   } catch (err) {
     if (err.code === 'ENOENT') {
       throw new Error(`Could not find state for '${user}'. Did you call login_user(browser, workerInfo, '${user}') in test.beforeAll()?`);
+    } else {
+      throw err;
     }
   }
-  return context;
 }
 
 export async function save_visual(page: Page) {