fix: add null check for post objects in usePostList hook

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);
       }
     });