ZO-5265: We need to set null as enum value for a nullable enum

https://json-schema.org/draft/2020-12/json-schema-validation\#name-enum

docs/api/api.yaml

@@ -894,7 +894,7 @@ components:
         access:
           type: string
           nullable: true
-          enum: [abo, registration, dynamic]
+          enum: [abo, registration, dynamic, null]
           description: "content access restriction"
         color:
           type: string