Handle favicon not found when auth is enabled

If Pages has access control enabled, it will trigger the auth flow for not found resources. This happens for favicon.ico

This is the intended behavior because we don't want to expose that a private domain/project exists. We could skip the auth redirection in https://gitlab.com/gitlab-org/gitlab-pages/-/blob/v1.23.0/internal/auth/auth.go#L116 for favicon but we will need to make sure we don't expose it for private projects.