Fix handling singleton resources without ID given in the URL

This fixes case when we have a singleton resource without ID given like `/account` which returns info for current logged in account. Without this fix it was raising exception `split` called on `nil`

btw. name `Preferences` is a bit unfortunate, it causes issues because it's uncountable. I could rename this model to `Account` everywhere or leave it like this, with configuring `ActiveSupport::Inflector` properly.