JS -- Add listener for sandbox events only if there are some actions

* When no actions then set it to null instead of empty object
* Even if a field has no actions, it needs to listen to events from the sandbox in order to be updated if an action changes something in it.

src/core/worker.js

@@ -525,6 +525,10 @@ class WorkerMessageHandler {
       return pdfManager.ensureDoc("fieldObjects");
     });
 
+    handler.on("HasJSActions", function (data) {
+      return pdfManager.ensureDoc("hasJSActions");
+    });
+
     handler.on("GetCalculationOrderIds", function (data) {
       return pdfManager.ensureDoc("calculationOrderIds");
     });