Make sure no error if page number is not set

reader/templates/reader/book_read/pdf_viewer.html

@@ -651,9 +651,11 @@ See https://github.com/adobe-type-tools/cmap-resources
 
                 PDFViewerApplication.eventBus.on("pagechanging", (e) => {
                     console.log('pagechanging, from ' + e.previous + ' to ' + e.pageNumber);
+                    {% if pagenumber %}
                         if (e.pageNumber === {{ pagenumber }}) {
                             return;
                         }
+                    {% endif %}
                     $.ajax(
                         "{% url 'update_progress' book_id=book.id %}",
                         {