be in sync with www (#6478)

Author: potatowagon

packages/lexical-react/flow/LexicalDraggableBlockPlugin.js.flow

@@ -10,7 +10,7 @@
 import * as React from 'react';
 
 type Props = $ReadOnly<{
-  anchorElem?: HTMLElement,
+  anchorElem?: ?HTMLElement,
   menuRef: React.RefObject<?HTMLElement>,
   targetLineRef: React.RefObject<?HTMLElement>,
   menuComponent: React.Node,