Fix hydration with empty spread attributes (#4647)

* Fix hydration of empty spread attributes

* add a regression test

* fix doctest

packages/web/src/hydration/hydrate.rs

@@ -301,9 +301,12 @@ impl WebsysDom {
                         let id = vnode
                             .mounted_dynamic_attribute(*id, dom)
                             .ok_or(VNodeNotInitialized)?;
+                        // We always need to hydrate the node even if the attributes are empty so we have
+                        // a mount for the node later. This could be spread attributes that are currently empty,
+                        // but will be filled later
+                        mounted_id = Some(id);
                         for attribute in attributes {
                             let value = &attribute.value;
-                            mounted_id = Some(id);
                             if let AttributeValue::Listener(_) = value {
                                 if attribute.name == "onmounted" {
                                     to_mount.push(id);