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/domain/Category.spec.ts

@@ -15,7 +15,7 @@ describe('Category', () => {
         const construct = () => new Category(5, name, [], [new CategoryStub(5)], []);
         // assert
         expect(construct).to.throw(expectedError);
-      });
+      }, { excludeNull: true, excludeUndefined: true });
     });
     it('throws when has no children', () => {
       const expectedError = 'A category must have at least one sub-category or script';