Fix nil behaviour

```
Failure: StandardEditionTranslationBlockContentConsistencyTest#test_editions_without_images_should_not_have_image_key_in_block_content [test/unit/app/models/standard_edition_translation_block_content_consistency_test.rb:45]: English translation should not have image key. Expected ["field_attribute", "body", "image"] to not include "image". bin/rails test test/unit/app/models/standard_edition_translation_block_content_consistency_test.rb:42
```

This test was failing after the tweak in the previous commit.
Why?

- When we call `BlockContent#assign_attributes`, it iterates every attribute in the schema and calls the setter for it.
- For "image" (type "object"), the code does `public_send("image=", values["image"])`.
- If values["image"] is missing, that's `nil` — and because the
  attributes class has `attribute :image`, `to_h` will include
  `"image" => nil`, which makes `block_content.keys` include "image".

So we've just had to add a `.compact` to filter out all `nil`s.

Author: ChrisBAshton

app/models/standard_edition/block_content.rb

@@ -91,7 +91,7 @@ def attributes_class_for(attribute_config)
       delegate :[], to: :attributes
 
       def to_h
-        attributes
+        attributes.compact
       end
     end
     attributes_class.set_temporary_name("Block content attributes")