Add empty favicon.ico

To avoid `ActionController::RoutingError` from headless chrome for Rails < 7.2.
Like the following:
```
Failures:

  1) User views ember app using route helper
     Failure/Error: raise ActionController::RoutingError, "No route matches [#{env['REQUEST_METHOD']}] #{env['PATH_INFO'].inspect}"

     ActionController::RoutingError:
       No route matches [GET] "/favicon.ico"
```