fixup! Implement and use HibernationManagerImpl

src/workerd/io/hibernation-manager.h

@@ -137,7 +137,7 @@ class HibernationManagerImpl final : public Worker::Actor::HibernationManager {
 
     // The following properties are moved from the `api::WebSocket` when the actor initiates
     // hibernation, and are moved back into the `api::WebSocket` when it is re-activated.
-    kj::Array<kj::byte> attachment;
+    kj::Maybe<kj::Array<kj::byte>> attachment;
     // Serialized attachment property of the api::WebSocket.
     // The `attachment` is read upon waking from hibernation, and written to upon hibernating.
     kj::Maybe<kj::String> url;