3d: Add BlogPageController

Add the BlogPageController that is referenced within
`my_module.routing.yml`.

We will need to return a render array, but for now let’s return an empty
array.

The test now passes as we are getting the expected response code.

web/modules/custom/my_module/src/Controller/BlogPageController.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Drupal\my_module\Controller;
+
+class BlogPageController {
+
+  public function __invoke(): array {
+    return [];
+  }
+
+}
+