Fix OpenAPI schema validation errors in Subscription model

Author: simukappu

lib/activity_notification/models/concerns/swagger/subscription_schema.rb

@@ -85,7 +85,8 @@ module Swagger::SubscriptionSchema #:nodoc:
                     },
                     subscribed_at: {
                       type: "string",
-                      format: "date-time"
+                      format: "date-time",
+                      nullable: true
                     }
                   }
                 }