Remove assumptions of attachment existence

This resolves a couple of scenarios where assumptions are made that
documents will have attachments and attachments with particular ids
exist. This is data that developers assume is the case (and most likely
is) but is beyond the guarantees that schemas make.

This mismatch can cause tests to fail when valid schemas are generated
which cannot be rendered.

app/presenters/content_item/attachments.rb

@@ -1,7 +1,7 @@
 module ContentItem
   module Attachments
     def attachment_details(attachment_id)
-      content_item["details"]["attachments"].find do |attachment|
+      content_item.dig("details", "attachments")&.find do |attachment|
         attachment["id"] == attachment_id
       end
     end

app/views/content_items/_attachments.html.erb

@@ -1,4 +1,6 @@
-<% if legacy_pre_rendered_documents.present? || attachments.any? %>
+<% attachments ||= [] %>
+<% attachment_details = attachments.filter_map { |id| @content_item&.attachment_details(id) } %>
+<% if legacy_pre_rendered_documents.present? || attachment_details.any? %>
   <section id="<%= title.parameterize %>">
     <%= render 'govuk_publishing_components/components/heading',
         text: title,
@@ -11,15 +13,14 @@
         <%= raw(legacy_pre_rendered_documents) %>
       <% end %>
     <% else %>
-      <% attachments.each do |attachment_id| %>
+      <% attachment_details.each do |details| %>
         <div class="attachment">
           <%= render 'govuk_publishing_components/components/attachment', {
             heading_level: 3,
-            attachment: @content_item.attachment_details(attachment_id)
+            attachment: details
           } %>
         </div>
       <% end %>
     <% end %>
-
   </section>
 <% end %>