Fixes mlModel graphql when user is nil

What does this MR do and why?

Fixes mlModel graphql when user is nil

When anonymous users would access a query on mlModels, they would crash even if they did have access to the endpoint. This fixes by using Ability.allowed? instead of user.can?

How to set up and validate locally

Example below:

1. In rails console enable the experiment fully

   Feature.enable(:model_registry)

2. Create a model

   project_id = 1
   Ml::FindOrCreateModelService.new(Project.find_by(id:project_id), "model_1").execute

3. Open a anonymous window, navigate to <gdk>/<project>/-/ml/models/<model_id>, then on candidates. A message 'This model has no candidates' should be displayed, instead of 500 error