fix(Various): Use empty properties object. Ensure $extends.

schema/Collection.schema.yaml

@@ -8,9 +8,9 @@ description: |
 # The [schema.org defintion](https://schema.org/Collection) adds
 # `collectionSize` but has not been added here because that feels
 # redudant e.g. a developer could use `collection.parts.length` instead
+properties: {}
 required:
   # Note that `parts` is defined in `CreativeWork` but made
   # a requirement here. A creative work need not be required to have
   # parts, but a collection should.
   - parts
-additionalProperties: false

schema/text/CodeBlock.schema.yaml

@@ -4,4 +4,4 @@ $extends: Code.schema.yaml
 role: secondary
 status: stable
 description: A code block.
-additionalProperties: false
+properties: {}

schema/text/ThematicBreak.schema.yaml

@@ -1,9 +1,8 @@
 title: ThematicBreak
 '@id': stencila:ThematicBreak
+$extends: ../Thing.schema.yaml
 role: secondary
-status: experimental
+status: stable
 description: |
   A thematic break, such as a scene change in a story, a transition to another topic, or a new document.
-required:
-  - type
-additionalProperties: false
+properties: {}