Add empty manifest.json

[AJGranowski]

Alternative to #11779

What are these changes?

This change adds an empty manifest.json file to the repo.

Why are these changes being made?

The first run of bin/docker_dev.sh causes both nginx and php-dusk fail.

php:

Exception
The manifest does not exist.
at app/Singletons/AssetsManifest.php:22
   18▕     {
   19▕         $manifestPath = public_path('assets/manifest.json');
   20▕
   21▕         if (!file_exists($manifestPath)) {
➜ 22▕             throw new Exception('The manifest does not exist.');
   23▕         }
   24▕
   25▕         $this->manifest = json_decode(file_get_contents($manifestPath), true);
   26▕     }
1   app/Providers/AppServiceProvider.php:96
    App\Singletons\AssetsManifest::__construct()

php-dusk:

php-dusk exited with code 157

nginx:

[emerg] host not found in upstream "php-dusk" in /etc/nginx/conf.d/default.conf:53
nginx exited with code 1

Successive runs pass and serve the website, suggesting a race condition in docker compose up on the manifest file existing.

[nanaya]

just for reference this specific change means git status will mark the file as modified "permanently" (until someone accidentally commit their local changes)

[AJGranowski]

Closing in favor of #11779