fix onload and onerror parsing error (#3707)

regression error induced by 299b123 where serde default got removed from `load_error` attribute of `ImageData`.

```diff
pub struct ImageData {
-     #[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
-     pub load_error: bool,
+     inner: Box<dyn HasImageData>,
}

// ...

+ struct SerializedImageData {
+     load_error: bool,
+ }
```

packages/html/src/events/image.rs

@@ -45,6 +45,7 @@ impl ImageData {
 /// A serialized version of ImageData
 #[derive(serde::Serialize, serde::Deserialize, Debug, PartialEq, Clone)]
 pub struct SerializedImageData {
+    #[cfg_attr(feature = "serialize", serde(default))]
     load_error: bool,
 }