refactor Content Block Document validation

because we have these lines

```
belongs_to :document, touch: true
...
accepts_nested_attributes_for :document
```

the `document` should never be nil when creating
an Edition.

lib/engines/content_block_manager/app/models/content_block_manager/content_block/edition/documentable.rb

@@ -12,19 +12,12 @@ module ContentBlock::Edition::Documentable
     end
 
     def block_type
-      document&.block_type || @block_type
+      @block_type ||= document&.block_type
     end
 
     def ensure_presence_of_document
-      if document.blank?
-        self.document = ContentBlock::Document.new(
-          content_id: create_random_id,
-          block_type: @block_type,
-          sluggable_string: title,
-        )
-      elsif document.new_record?
+      if document.new_record?
         document.content_id = create_random_id if document.content_id.blank?
-        document.block_type = @block_type if document.block_type.blank?
         document.sluggable_string = title if document.sluggable_string.blank?
       end
     end

lib/engines/content_block_manager/test/unit/app/validators/organisation_validator_test.rb

@@ -4,7 +4,7 @@ class ContentBlockManager::OrganisationValidatorTest < ActiveSupport::TestCase
   extend Minitest::Spec::DSL
 
   test "it validates presence of a lead organisation" do
-    content_block_edition = build(:content_block_edition, organisation: nil)
+    content_block_edition = build(:content_block_edition, organisation: nil, document: build(:content_block_document, :email_address))
 
     assert_equal false, content_block_edition.valid?
     assert_equal ["cannot be blank"], content_block_edition.errors.messages[:lead_organisation]