fix for virtual-scroll undefined method call

fixes https://github.com/wenzhixin/bootstrap-table/issues/5997

src/constants/index.js

@@ -337,6 +337,9 @@ const DEFAULTS = {
   },
   onTogglePagination (newState) {
     return false
+  },
+  onVirtualScroll (newState) {
+    return false
   }
 }