fix(getPageContentText): add null check for postContent before accessing length property

Ensure postContent exists before checking its length property to prevent
potential runtime errors when postContent is null or undefined.

lib/notion/getPageContentText.js

@@ -168,7 +168,7 @@ export function getPageContentText(post, pageBlockMap) {
   const postContent = post.content
   let contentTextList = []
   // 防止搜到加密文章的内容
-  if (postContent.length > 0 && !post.password) {
+  if (postContent && postContent.length > 0 && !post.password) {
     for (const postContentId of postContent) {
       const blockContentText = getBlockContentText(postContentId)
       if (blockContentText) {