Don't show unpersisted `has_one` associations

Created by: rwehresmann

Showing unpersisted has_one associations can be misleading and break the application is some cases.

Let's consider the following example:

class Product < ApplicationRecord
  has_one :product_meta_tag

  def product_meta_tag
    super || build_product_meta_tag
  end
end

That opens for a scenario where a product.product_meta_tag will return a non-persisted instance of a ProductMetaTag. In the _show partial we're just checking if field.data (i.e., ProductMetaTag instance) exists to display it, and a non-persisted instance would be displayed. The link displayed in the partial (link_to( field.display_associated_resource, [namespace, field.data]) would be pointing to the index page of product meta tags, what is not desired. Also, in the absence of an index route, the page would break.

The solution: just check if data is persisted to display it or not.