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/shared/Stubs/ExpressionParserStub.ts

@@ -20,8 +20,9 @@ export class ExpressionParserStub implements IExpressionParser {
 
   public findExpressions(code: string): IExpression[] {
     this.callHistory.push(code);
-    if (this.results.has(code)) {
-      return [...this.results.get(code)];
+    const foundResult = this.results.get(code);
+    if (foundResult) {
+      return [...foundResult];
     }
     return [];
   }