Declare the js context as nullable in skwasm surface callback (#45810)

Declaring this as non-nullable causes breakage with an incoming JS interop change from the SDK. Marking this as nullable fixes the issue.

This doesn't actually change any behavior. The breakages were in `test/ui/image_golden_test.dart`. No changes to the test are required (they did catch the issue when the dart roll happened).

Author: eyebrowsoffire

lib/web_ui/lib/src/engine/skwasm/skwasm_impl/surface.dart

@@ -78,7 +78,7 @@ class SkwasmSurface {
     return completer.future;
   }
 
-  void _callbackHandler(JSNumber callbackId, JSNumber context, JSAny jsContext) {
+  void _callbackHandler(JSNumber callbackId, JSNumber context, JSAny? jsContext) {
     final Completer<JSAny> completer = _pendingCallbacks.remove(callbackId.toDartInt)!;
     if (jsContext.isUndefinedOrNull) {
       completer.complete(context);

lib/web_ui/skwasm/library_skwasm_support.js

@@ -39,8 +39,9 @@ mergeInto(LibraryManager.library, {
               object.close();
             }
             associatedObjectsMap.delete(data.pointer);
+            return;
           default:
-            console.warn('unrecognized skwasm message');
+            console.warn(`unrecognized skwasm message: ${skwasmMessage}`);
         }
       };
       if (!threadId) {