Fix: update logseq.Editor.getPageBlocksTree's return type. (#12212)

Changes the logseq.Editor.getPageBlocksTree's return type from
Promise<Array<BlockEntity>> to Promise<Array<BlockEntity> | null>.

Closes #12211.

Co-authored-by: Charlie <[email protected]>

libs/src/LSPlugin.ts

@@ -657,7 +657,7 @@ export interface IEditorProxy extends Record<string, any> {
    *
    * @param srcPage - the page name or uuid
    */
-  getPageBlocksTree: (srcPage: PageIdentity) => Promise<Array<BlockEntity>>
+  getPageBlocksTree: (srcPage: PageIdentity) => Promise<Array<BlockEntity> | null>
 
   /**
    * get all page/block linked references