fix: add null check for post objects in usePostList hook (#752)

Adds defensive null checks when accessing post properties in the posts
forEach loop to prevent potential errors in the MFE discussion sidebar.
This addresses the issue reported in #751.

src/discussions/posts/data/hooks.js

@@ -11,9 +11,9 @@ const usePostList = (ids) => {
 
   const sortedIds = useMemo(() => {
     posts.forEach((post) => {
-      if (post.pinned) {
+      if (post && post.pinned) {
         pinnedPostsIds.push(post.id);
-      } else {
+      } else if (post) {
         unpinnedPostsIds.push(post.id);
       }
     });