Ensure that the data is loaded, in the "GetPageJSActions" message handler

Similar to all other data accesses, note e.g. the "GetDocJSActions" handler just above, we need to ensure that a `MissingDataException` isn't propagated to the main-thread if this data is accessed while the PDF document is still loading.

src/core/worker.js

@@ -487,7 +487,7 @@ class WorkerMessageHandler {
 
     handler.on("GetPageJSActions", function ({ pageIndex }) {
       return pdfManager.getPage(pageIndex).then(function (page) {
-        return page.jsActions;
+        return pdfManager.ensure(page, "jsActions");
       });
     });