Don't show failed translation message for missing document types

With the update to Whitehall that allows adding any GOV.UK page to a
document collection there is the potential users could add documents
with types the Government Frontend knows nothing about. For these cases
this will return a nil value rather than raise a failed translation
message.

At this point in time it seems unlikely users would use a document type
that is not known about as the expected usage is Content Publisher
document types. However there is nothing to prevent wider usage of
document types.

app/presenters/document_collection_presenter.rb

@@ -41,7 +41,9 @@ def group_document_links(group, group_index)
         },
         metadata: {
           public_updated_at: Time.zone.parse(link["public_updated_at"]),
-          document_type: I18n.t("content_item.schema_name.#{link['document_type']}", count: 1)
+          document_type: I18n.t("content_item.schema_name.#{link['document_type']}",
+                                count: 1,
+                                default: nil),
         },
       }
     end

test/presenters/document_collection_presenter_test.rb

@@ -73,6 +73,21 @@ class PresentedDocumentCollection < TestCase
 
       assert_equal documents, presented_item.group_document_links({ "documents" => [document_ids.first] }, 0)
     end
+
+    test 'it handles the document type lacking a translation' do
+      schema_data = schema_item
+
+      document = schema_data["links"]["documents"].first.tap do |link|
+        link["document_type"] = "non-existant"
+      end
+
+      grouped = present_example(schema_data).group_document_links(
+        { "documents" => [document["content_id"]] },
+        0,
+      )
+
+      assert_nil grouped.first[:metadata][:document_type]
+    end
   end
 
   class GroupWithMissingDocument < TestCase