chore(spec): replace ugly hack with another hack for optional properties

public/openapi/components/schemas/TopicObject.yaml

@@ -1,5 +1,6 @@
 TopicObject:
-  type: object
+  allOf:
+    - type: object
       properties:
         tid:
           type: number
@@ -221,38 +222,13 @@ TopicObject:
             description: HTML injected into the theme
         index:
           type: number
+    - type: object
+      description: Optional properties that may or may not be present (except for `tid`, which is always present, and is only here as a hack to pass validation)
+      properties:
+        tid:
+          type: number
+          description: A topic identifier
         thumb:
           type: string
       required:
         - tid
-    - uid
-    - cid
-    - mainPid
-    - title
-    - slug
-    - timestamp
-    - lastposttime
-    - postcount
-    - viewcount
-    - teaserPid
-    - upvotes
-    - downvotes
-    - deleted
-    - locked
-    - pinned
-    - deleterUid
-    - titleRaw
-    - timestampISO
-    - lastposttimeISO
-    - votes
-    - category
-    - user
-    - teaser
-    - tags
-    - isOwner
-    - ignored
-    - unread
-    - bookmark
-    - unreplied
-    - icons
-    - index

public/openapi/read/topic/topic_id.yaml

@@ -403,6 +403,17 @@ get:
                     type: number
                   loggedInUser:
                     $ref: ../../components/schemas/UserObject.yaml#/UserObject
+              - type: object
+                description: Optional properties that may or may not be present (except for `tid`, which is always present, and is only here as a hack to pass validation)
+                properties:
+                  tid:
+                    type: number
+                    description: A topic identifier
+                  thumb:
+                    type: string
+                    description: An uploaded topic thumbnail
+                required:
+                  - tid
               - $ref: ../../components/schemas/Pagination.yaml#/Pagination
               - $ref: ../../components/schemas/Breadcrumbs.yaml#/Breadcrumbs
               - $ref: ../../components/schemas/CommonProps.yaml#/CommonProps