Fix reducer error when conversation has no last status in web UI (#9173)

Fix #9170

app/javascript/mastodon/reducers/conversations.js

@@ -21,7 +21,7 @@ const conversationToMap = item => ImmutableMap({
   id: item.id,
   unread: item.unread,
   accounts: ImmutableList(item.accounts.map(a => a.id)),
-  last_status: item.last_status.id,
+  last_status: item.last_status ? item.last_status.id : null,
 });
 
 const updateConversation = (state, item) => state.update('items', list => {