fix: Remove incorrect nullable annotations from ai_subtopic.id and ai_topics

ai_subtopic.id is always present, and ai_topics returns {} not null.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: niall-trinder

descriptions/0/api.intercom.io.yaml

@@ -16102,7 +16102,6 @@ components:
           type: integer
           description: The ID of the subtopic.
           example: 42
-          nullable: true
     ai_topic:
       title: AI Topic
       type: object
@@ -18185,7 +18184,6 @@ components:
           "$ref": "#/components/schemas/linked_object_list"
         ai_topics:
           "$ref": "#/components/schemas/ai_topics"
-          nullable: true
         ai_agent_participated:
           type: boolean
           description: Indicates whether the AI Agent participated in the conversation.