Fix Swagger spec for custom topic values

Author: GeorgDangl

swagger.yaml

@@ -2438,11 +2438,12 @@ components:
       properties:
         id:
           type: string
-          nullable: true
           description: "Refers to the id of the custom field in the project extensions."
         value:
-          type: string
-          nullable: true
+          type: array
+          items:
+            type: string
+            nullable: true
     # viewpoint schemas
     bitmap_GET:
       type: object