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/unit/application/Context/State/CategoryCollectionState.spec.ts

@@ -92,7 +92,11 @@ describe('CategoryCollectionState', () => {
       // act
       let actualScript: IScript | undefined;
       sut.filter.filterChanged.on((result) => {
-        [actualScript] = result.filter?.scriptMatches ?? [undefined];
+        result.visit({
+          onApply: (filter) => {
+            [actualScript] = filter.scriptMatches;
+          },
+        });
       });
       sut.filter.applyFilter(scriptNameFilter);
       // assert