fix: mark update_internal_article_request as non-nullable

The schema was incorrectly marked as nullable, causing Java SDK generation
to fail with "cannot find symbol: method isPresent()" errors.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: marcmlc

fern/unstable-openapi-overrides.yml

@@ -107,6 +107,7 @@ components:
         ticket_attributes: null
     update_internal_article_request:
       x-fern-type-name: UpdateInternalArticleRequestBody
+      nullable: false
     reply_conversation_request:
       x-fern-type-name: ReplyConversationRequestBody
     update_article_request: