Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

tests/checks/external-urls/StatusChecker/ExponentialBackOffRetryHandler.ts

@@ -26,6 +26,9 @@ function shouldRetry(status: IUrlStatus) {
   if (status.error) {
     return true;
   }
+  if (status.code === undefined) {
+    return true;
+  }
   return isTransientError(status.code)
     || status.code === 429; // Too Many Requests
 }