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.

src/domain/ICategory.ts

@@ -5,8 +5,8 @@ import { IDocumentable } from './IDocumentable';
 export interface ICategory extends IEntity<number>, IDocumentable {
   readonly id: number;
   readonly name: string;
-  readonly subCategories?: ReadonlyArray<ICategory>;
-  readonly scripts?: ReadonlyArray<IScript>;
+  readonly subCategories: ReadonlyArray<ICategory>;
+  readonly scripts: ReadonlyArray<IScript>;
   includes(script: IScript): boolean;
   getAllScriptsRecursively(): ReadonlyArray<IScript>;
 }