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/ExpressionStub.ts

@@ -11,7 +11,7 @@ export class ExpressionStub implements IExpression {
 
   public parameters: IReadOnlyFunctionParameterCollection = new FunctionParameterCollectionStub();
 
-  private result: string = undefined;
+  private result = `[${ExpressionStub.name}] result`;
 
   public withParameters(parameters: IReadOnlyFunctionParameterCollection) {
     this.parameters = parameters;