Silently return nil for session when client supplies unknown session id

When using ActiveRecord for session storage, it raises `ActiveRecord::RecordNotFound` for nonexistent session ids. Gotta handle that.
Could do the following instead, a missing record isn't *that* exceptional :)

```ruby
if shop = Shop.where(id: id).first
  ShopifyAPI::Session.new(shop.domain, shop.token)
end
```